Light-emitting element, light-emitting device, electronic device, and lighting device

ABSTRACT

A light-emitting layer, which is a stack of a first light-emitting layer and a second light-emitting layer, is provided between an anode and a cathode. The first light-emitting layer is formed on the anode side and contains a first light-emitting substance converting triplet excitation energy into light emission, a first organic compound having an electron-transport property, and a second organic compound having a hole-transport property. The second light-emitting layer contains a second light-emitting substance converting triplet excitation energy into light emission, the second organic compound, and a third organic compound having an electron-transport property. The first organic compound has a higher LUMO level than the third organic compound. The first light-emitting substance emits light with a wavelength shorter than that of light emitted from the second light-emitting substance. The first and the second organic compounds form an exciplex. The second and the third organic compounds form an exciplex.

BACKGROUND OF THE INVENTION

1. Field of the Invention

One embodiment of the present invention relates to a light-emitting element in which an organic compound capable of providing light emission by application of an electric field is provided between a pair of electrodes, and also relates to a light-emitting device, an electronic device, and a lighting device including such a light-emitting element.

2. Description of the Related Art

Light-emitting elements including an organic compound as a luminous body, which have features such as thinness, lightness, high-speed response, and DC driving at low voltage, are expected to be applied to next-generation flat panel displays. In particular, display devices in which light-emitting elements are arranged in a matrix are considered to have advantages of a wide viewing angle and high visibility over conventional liquid crystal display devices.

It is said that the light emission mechanism of a light-emitting element is as follows: when a voltage is applied between a pair of electrodes with an EL layer including a luminous body provided therebetween, electrons injected from the cathode and holes injected from the anode recombine in the light emission center of the EL layer to form molecular excitons, and energy is released and light is emitted when the molecular excitons relax to the ground state. A singlet excited state and a triplet excited state are known as the excited states, and it is thought that light emission can be obtained through either of the excited states.

In order to improve element characteristics of such light-emitting elements, improvement of an element structure, development of a material, and the like have been actively carried out (see, for example, Patent Document 1).

Reference

-   [Patent Document 1] Japanese Published Patent Application No.     2010-182699

SUMMARY OF THE INVENTION

However, it is said that the light extraction efficiency of a light-emitting element at present is approximately 20% to 30%. Even considering light absorption by a reflective electrode and a transparent electrode, the external quantum efficiency of a light-emitting element including a phosphorescent compound has a limit of approximately 25% at most.

In one embodiment of the present invention, a light-emitting element with high external quantum efficiency is provided. In another embodiment of the present invention, a light-emitting element having a long lifetime is provided.

One embodiment of the present invention is a light-emitting element including a light-emitting layer between a pair of electrodes (anode and cathode). The light-emitting layer has a stacked-layer structure including a first light-emitting layer and a second light-emitting layer. The first light-emitting layer is formed on the anode side and contains at least a first light-emitting substance (guest material) converting triplet excitation energy into light emission, a first organic compound (host material) having an electron-transport property, and a second organic compound (assist material) having a hole-transport property. The second light-emitting layer contains at least a second light-emitting substance (guest material) converting triplet excitation energy into light emission, a third organic compound (host material) having an electron-transport property, and the second organic compound (assist material) having a hole-transport property. In the first light-emitting layer, a combination of the first organic compound (host material) and the second organic compound (assist material) forms an exciplex, and in the second light-emitting layer, a combination of the third organic compound (host material) and the second organic compound (assist material) forms an exciplex.

Another embodiment of the present invention is a light-emitting element including a light-emitting layer between an anode and a cathode, a hole-transport layer between the anode and the light-emitting layer, and an electron-transport layer between the cathode and the light-emitting layer. The light-emitting layer is a stack of a first light-emitting layer and a second light-emitting layer. The first light-emitting layer is formed in contact with the hole-transport layer and contains at least a first light-emitting substance converting triplet excitation energy into light emission, a first organic compound having an electron-transport property, and a second organic compound having a hole-transport property. The second light-emitting layer is formed in contact with the electron-transport layer and contains at least a second light-emitting substance converting triplet excitation energy into light emission, a third organic compound having an electron-transport property, and the second organic compound having a hole-transport property. In the first light-emitting layer, a combination of the first organic compound (host material) and the second organic compound (assist material) forms an exciplex, and in the second light-emitting layer, a combination of the third organic compound (host material) and the second organic compound (assist material) forms an exciplex.

Note that in each of the above structures, the emission wavelength of the exciplex formed by the first organic compound (host material) and the second organic compound (assist material) in the first light-emitting layer is located on the longer wavelength side with respect to the emission wavelength (fluorescent wavelength) of each of the first and second organic compounds (host and assist materials). Therefore, by formation of the exciplex, the fluorescence spectrum of the first organic compound (host material) and the fluorescence spectrum of the second organic compound (assist material) can be converted into an emission spectrum which is located on the longer wavelength side. Further, the emission wavelength of the exciplex formed by the third organic compound (host material) and the second organic compound (assist material) in the second light-emitting layer is located on the longer wavelength side with respect to the emission wavelength (fluorescent wavelength) of each of the third and second organic compounds (host and assist materials). Therefore, by formation of the exciplex, the fluorescence spectrum of the third organic compound (host material) and the fluorescence spectrum of the second organic compound (assist material) can be converted into an emission spectrum which is located on the longer wavelength side.

Note that in each of the above structures, in the first light-emitting layer, an exciplex is formed from an anion of the first organic compound and a cation of the second organic compound, and in the second light-emitting layer, an exciplex is formed from an anion of the third organic compound and a cation of the second organic compound.

In the above structures, the first light-emitting layer and the second light-emitting layer contain the same second organic compound (assist material), and the lowest unoccupied molecular orbital level (LUMO level) of the first organic compound (host material) contained in the first light-emitting layer is higher than that of the third organic compound (host material) contained in the second light-emitting layer.

In addition, in the above structures, the first light-emitting substance contained in the first light-emitting layer is a substance which emits light with a wavelength shorter than that of light emitted from the second light-emitting substance contained in the second light-emitting layer.

In the above structures, the first light-emitting substance and the second light-emitting substance are light-emitting substances which convert triplet excitation energy into light emission, and phosphorescent compounds such as an organometallic complex, or a material emitting thermally activated delayed fluorescence, i.e., a thermally activated delayed fluorescence (TADF) material, can be used. Further, the first organic compound and the third organic compound are mainly electron-transport materials with an electron mobility of 10⁻⁶ cm²/Vs or more, specifically, π-electron deficient heteroaromatic compounds. The second organic compound is mainly a hole-transport material with a hole mobility of 10⁻⁶ cm²/Vs or more, specifically, a π-electron rich heteroaromatic compound or an aromatic amine compound.

Further, the present invention includes, in its scope, electronic devices and lighting devices including light-emitting devices, as well as light-emitting devices including light-emitting elements. The light-emitting device in this specification refers to an image display device and a light source (e.g., a lighting device). In addition, the light-emitting device includes all the following modules: a module in which a connector, such as a flexible printed circuit (FPC) or a tape carrier package (TCP), is attached to a light-emitting device; a module in which a printed wiring board is provided at the end of a TCP; and a module in which an integrated circuit (IC) is directly mounted on a light-emitting element by a chip-on-glass (COG) method.

Note that in a light-emitting element that is one embodiment of the present invention, an exciplex can be formed in each of the first light-emitting layer and the second light-emitting layer which are included in the light-emitting layer; thus, the light-emitting element can have high energy transfer efficiency and high external quantum efficiency.

Further, in a light-emitting element that is one embodiment of the present invention, owing to the above-described element structure, the exciplex formed in the first light-emitting layer has higher excitation energy than the exciplex formed in the second light-emitting layer; therefore, when a substance emitting light with a wavelength shorter than that of light emitted from the second light-emitting substance (guest material) which is contained in the second light-emitting layer and converts triplet excitation energy into light emission is used as the first light-emitting substance (guest material) which is contained in the first light-emitting layer and converts triplet excitation energy into light emission, the first light-emitting layer and the second light-emitting layer can provide light emission at the same time. Further, part of excitation energy of the exciplex formed in the first light-emitting layer which does not contribute to light emission can be utilized as excitation energy for the second light-emitting substance (guest material) which converts triplet excitation energy into light emission in the second light-emitting layer. Accordingly, emission efficiency in the light-emitting element can be further increased.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B illustrate a concept of one embodiment of the present invention.

FIG. 2 illustrates a concept of one embodiment of the present invention.

FIG. 3 show calculation results according to one embodiment of the present invention.

FIGS. 4A1, 4A2, 4B1, 4B2, 4C1, and 4C2 show calculation results according to one embodiment of the present invention.

FIG. 5 illustrates a structure of a light-emitting element.

FIGS. 6A and 6B each illustrate a structure of a light-emitting element.

FIG. 7 illustrates a light-emitting device.

FIGS. 8A and 8B illustrate a light-emitting device.

FIGS. 9A to 9D illustrate electronic devices.

FIGS. 10A to 10C illustrate an electronic device.

FIG. 11 illustrates lighting devices.

FIG. 12 illustrates a light-emitting element.

FIG. 13 shows current density-luminance characteristics of a light-emitting element 1.

FIG. 14 shows voltage-luminance characteristics of a light-emitting element 1.

FIG. 15 shows luminance-current efficiency characteristics of a light-emitting element 1.

FIG. 16 shows voltage-current characteristics of a light-emitting element 1.

FIG. 17 shows an emission spectrum of a light-emitting element 1.

FIG. 18 shows emission spectra of substances used in a light-emitting element 1.

FIG. 19 shows emission spectra of substances used in a light-emitting element 1.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described below with reference to the drawings. Note that the present invention is not limited to the following description, and various changes and modifications can be made without departing from the spirit and scope of the invention. Therefore, the present invention should not be construed as being limited to the description in the following embodiments.

(Elementary Process of Light Emission in Light-Emitting Element)

First, a description is given of general elementary processes of light emission in a light-emitting element which uses a light-emitting substance converting triplet excitation energy into light emission (e.g., a phosphorescent compound or a thermally activated delayed fluorescence (TADF) material) as a guest material. Note that a molecule providing excitation energy is referred to as a host molecule, while a molecule receiving the excitation energy is referred to as a guest molecule.

(1) The case where an electron and a hole recombine in a guest molecule, and the guest molecule is put in an excited state (direct recombination process).

(1-1) When the excited state of the guest molecule is a triplet excited state, the guest molecule emits phosphorescence.

(1-2) When the excited state of the guest molecule is a singlet excited state, the guest molecule in the singlet excited state undergoes intersystem crossing to a triplet excited state and emits phosphorescence.

In other words, in the direct recombination process in (1), as long as the efficiency of intersystem crossing and the phosphorescence quantum yield of the guest molecule are high, high emission efficiency can be obtained. Note that the T₁ level of the host molecule is preferably higher than the T₁ level of the guest molecule.

(2) The case where an electron and a hole recombine in a host molecule and the host molecule is put in an excited state (energy transfer process).

(2-1) When the excited state of the host molecule is a triplet excited state and the T₁ level of the host molecule is higher than the T₁ level of the guest molecule, excitation energy is transferred from the host molecule to the guest molecule, and thus the guest molecule is put in a triplet excited state. The guest molecule in the triplet excited state emits phosphorescence. Note that energy transfer from the T₁ level of the host molecule to a singlet excitation energy level (S₁ level) of the guest molecule is forbidden unless the host molecule emits phosphorescence, and is unlikely to be a main energy transfer process; therefore, a description thereof is omitted here. In other words, energy transfer from the host molecule in the triplet excited state (3H*) to the guest molecule in the triplet excited state (3G*) is important as represented by Formula (2-1) below (where 1G represents the singlet ground state of the guest molecule and 1H represents the singlet ground state of the host molecule). 3H*+1G→1H+3G*  (2-1)

(2-2) When the excited state of the host molecule is a singlet excited state and the S₁ level of the host molecule is higher than the S₁ level and T₁ level of the guest molecule, excitation energy is transferred from the host molecule to the guest molecule, and thus, the guest molecule is put in a singlet excited state or a triplet excited state.

The guest molecule in the triplet excited state emits phosphorescence. In addition, the guest molecule in the singlet excited state undergoes intersystem crossing to a triplet excited state, and emits phosphorescence.

In other words, there can be a process where energy is transferred from the host molecule in the singlet excited state (1H*) to the guest molecule in the singlet excited state (1G*) and then the guest molecule is put in the triplet excited state (3G*) by intersystem crossing, as represented by Formula (2-2A) below, and a process where energy is directly transferred from the host molecule in the singlet excited state (1H*) to the guest molecule in the triplet excited state (3G*), as represented by Formula (2-2B) below. 1H*+1G→1H+1G*→(Intersystem crossing)→1H+3G*  (2-2A) 1H*+1G→1H+3G*  (2-2B)

When all the energy transfer processes described above in (2) occur efficiently, both the triplet excitation energy and the singlet excitation energy of the host molecule are efficiently converted into the triplet excited state (3G*) of the guest molecule. Thus, high-efficiency light emission is possible. In contrast, before the excitation energy of the host molecule is transferred to the guest molecule, when the host molecule itself is deactivated by emitting the excitation energy as light or heat, the emission efficiency decreases.

Next, factors controlling the above-described processes of intermolecular energy transfer between the host molecule and the guest molecule are described. As mechanisms of the intermolecular energy transfer, the following two mechanisms are proposed.

One mechanism is Förster mechanism (dipole-dipole interaction) in which energy transfer does not require direct contact between molecules and energy is transferred through a resonant phenomenon of dipolar oscillation between a host molecule and a guest molecule. By the resonant phenomenon of dipolar oscillation, the host molecule provides energy to the guest molecule, and thus, the host molecule is put in a ground state and the guest molecule is put in an excited state. Note that the rate constant k_(h*→g) of Förster mechanism is expressed by Formula (1).

$\begin{matrix} \left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack & \; \\ {k_{h^{*}\rightarrow g} = {\frac{9000\mspace{11mu} c^{4}K^{2}\phi\;\ln\mspace{11mu} 10}{128\pi^{5}n^{4}N\;\tau\; R^{6}}{\int{\frac{{f_{h}^{\prime}(v)}{ɛ_{g}(v)}}{v^{4}}{\mathbb{d}v}}}}} & (1) \end{matrix}$

In Formula (1), v denotes a frequency, f′_(h)(v) denotes a normalized emission spectrum of a host molecule (a fluorescence spectrum in energy transfer from a singlet excited state, and a phosphorescence spectrum in energy transfer from a triplet excited state), ε_(g)(v) denotes a molar absorption coefficient of a guest molecule, N denotes Avogadro's number, n denotes a refractive index of a medium, R denotes an intermolecular distance between the host molecule and the guest molecule, τ denotes a measured lifetime of an excited state (fluorescence lifetime or phosphorescence lifetime), c denotes the speed of light, φ denotes a luminescence quantum yield (a fluorescence quantum yield in energy transfer from a singlet excited state, and a phosphorescence quantum yield in energy transfer from a triplet excited state), and K² denotes a coefficient (0 to 4) of orientation of a transition dipole moment between the host molecule and the guest molecule. Note that K²=⅔ in random orientation.

The other is Dexter mechanism (electron exchange interaction) in which a host molecule and a guest molecule are close to a contact effective range where their orbitals overlap, and the host molecule in an excited state and the guest molecule in a ground state exchange their electrons, which leads to energy transfer. Note that the rate constant k_(h*→g) of Dexter mechanism is expressed by Formula (2).

$\begin{matrix} \left\lbrack {{Formula}\mspace{14mu} 2} \right\rbrack & \; \\ {k_{h^{*}\rightarrow g} = {\left( \frac{2\pi}{h} \right)K^{\prime 2}{\exp\left( {- \frac{2R}{L}} \right)}{\int{{f_{h}^{\prime}(v)}{ɛ_{g}^{\prime}(v)}{\mathbb{d}v}}}}} & (2) \end{matrix}$

In Formula (2), h denotes a Planck constant, K′ denotes a constant having an energy dimension, v denotes a frequency, f′_(h)(v) denotes a normalized emission spectrum of a host molecule (a fluorescence spectrum in energy transfer from a singlet excited state, and a phosphorescence spectrum in energy transfer from a triplet excited state), ε′_(g)(v) denotes a normalized absorption spectrum of a guest molecule, L denotes an effective molecular radius, and R denotes an intermolecular distance between the host molecule and the guest molecule.

Here, the efficiency of energy transfer from the host molecule to the guest molecule (energy transfer efficiency Φ_(ET)) is thought to be expressed by Formula (3). In the formula, k_(r) denotes a rate constant of a light-emission process (fluorescence in energy transfer from a singlet excited state, and phosphorescence in energy transfer from a triplet excited state) of a host molecule, k_(n) denotes a rate constant of a non-light-emission process (thermal deactivation or intersystem crossing) of a host molecule, and τ denotes a measured lifetime of an excited state of a host molecule.

$\begin{matrix} \left\lbrack {{Formula}\mspace{14mu} 3} \right\rbrack & \; \\ {\Phi_{ET} = {\frac{k_{h^{*}\rightarrow g}}{k_{r} + k_{n} + k_{h^{*}\rightarrow g}} = \frac{k_{h^{*}\rightarrow g}}{\left( \frac{1}{\tau} \right) + k_{h^{*}\rightarrow g}}}} & (3) \end{matrix}$

According to Formula (3), it is found that the energy transfer efficiency Φ_(ET) can be increased by increasing the rate constant k_(h*→g) of energy transfer so that another competing rate constant k_(r)+k_(n)(=1/τ) becomes relatively small.

(Energy Transfer Efficiency in (2-1))

Here, the energy transfer process in (2-1) is considered first. Since Förster mechanism (Formula (1)) is forbidden in this case, only Dexter mechanism (Formula (2)) should be considered. According to Formula (2), in order to increase the rate constant k_(h*→g), it is preferable that an emission spectrum of a host molecule (here, a phosphorescence spectrum because energy transfer from a triplet excited state is discussed) largely overlap with an absorption spectrum of a guest molecule (absorption corresponding to direct transition from a singlet ground state to a triplet excited state).

In one embodiment of the present invention, a light-emitting substance converting triplet excitation energy into light emission (e.g., a phosphorescent compound or a thermally activated delayed fluorescence (TADF) material) is used as a guest material. In an absorption spectrum of the phosphorescent compound, absorption corresponding to direct transition from a singlet ground state to a triplet excited state is observed in some cases, which is an absorption band on the longest wavelength side. In particular, light-emitting iridium complexes have a broad absorption band at around 500 nm to 600 nm as the absorption band on the longest wavelength side (as a matter of fact, the broad absorption band can be on a shorter or longer wavelength side depending on emission wavelengths). This absorption band is mainly based on a triplet MLCT (metal to ligand charge transfer) transition. Note that it is considered that the absorption band also includes absorptions based on a triplet π-π* transition and a singlet MLCT transition, and that these absorptions overlap each other to form a broad absorption band on the longest wavelength side in the absorption spectrum. In other words, the difference between the lowest singlet excited state and the lowest triplet excited state is small, and absorptions based on these states overlap each other to form a broad absorption band on the longest wavelength side in the absorption spectrum. Therefore, when an organometallic complex (especially iridium complex) is used as the guest material, the broad absorption band on the longest wavelength side largely overlaps with the phosphorescence spectrum of the host material as described above, whereby the rate constant k_(h*→g) can be increased and energy transfer efficiency can be increased.

Furthermore, a fluorescent compound is generally used as the host material; thus, a phosphorescence lifetime (τ) is a millisecond or longer which is extremely long (i.e., k_(r)+k_(n) is low). This is because the transition from the triplet excited state to the ground state (singlet) is a forbidden transition. Formula (3) shows that this is favorable to energy transfer efficiency Φ_(ET).

The above description also suggests that energy transfer from the host material in the triplet excited state to the guest material in the triplet excited state, i.e., the process in Formula (2-1), generally tends to occur as long as the phosphorescence spectrum of the host material overlaps with the absorption spectrum corresponding to the direct transition of the guest material from the singlet ground state to the triplet excited state.

(Energy Transfer Efficiency in (2-2))

Next, the energy transfer process in (2-2) is considered. The process in Formula (2-2A) is affected by the efficiency of intersystem crossing of the guest material. Therefore, in order to maximize emission efficiency, the process in Formula (2-2B) is considered to be important. Since Dexter mechanism (Formula (2)) is forbidden in this case, only Förster mechanism (Formula (1)) should be considered.

When τ is eliminated from Formula (1) and Formula (3), it can be said that the energy transfer efficiency Φ_(ET) is higher when the quantum yield φ (here, a fluorescent quantum yield because energy transfer from a singlet excited state is discussed) is higher. However, in practice, a more important factor is that the emission spectrum of the host molecule (here, a fluorescence spectrum because energy transfer from a singlet excited state is discussed) largely overlaps with the absorption spectrum of the guest molecule (absorption corresponding to the direct transition from the singlet ground state to the triplet excited state) (note that it is preferable that the molar absorption coefficient of the guest molecule be also high). This means that the fluorescence spectrum of the host material overlaps with the absorption band of the phosphorescent compound used as the guest material which is on the longest wavelength side.

However, this has conventionally been very difficult to achieve. The reason is that, in order to enable both of the above-described processes (2-1) and (2-2) to occur efficiently, it is clear from the above discussion that the host material should be designed so as to have not only its phosphorescence spectrum but also its fluorescence spectrum overlapping with the absorption band of the guest material which is on the longest wavelength side. In other words, the host material should be designed so as to have its fluorescence spectrum in a position similar to that of the phosphorescence spectrum.

However, in general, the S₁ level differs greatly from the T₁ level (S₁ level>T₁ level); therefore, the fluorescence emission wavelength also differs greatly from the phosphorescence emission wavelength (fluorescence emission wavelength<phosphorescence emission wavelength). For example, 4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP), which is commonly used as a host material in a light-emitting element including a phosphorescent compound, has a phosphorescence spectrum at around 500 nm and has a fluorescence spectrum at around 400 nm, which are largely different by about 100 nm. This example also shows that it is extremely difficult to design a host material so as to have its fluorescence spectrum in a position similar to that of its phosphorescence spectrum. Therefore, it is very important to improve efficiency in energy transfer from the host material in the singlet excited state to the guest material.

Therefore, one embodiment of the present invention provides a useful technique which can overcome such a problem of the efficiency of the energy transfer from the host material in the singlet excited state to the guest material. Specific embodiments thereof will be described below.

(Embodiment 1)

In this embodiment, a structural concept of a light-emitting element in one embodiment of the present invention and a specific structure of the light-emitting element will be described. Note that a light-emitting element in one embodiment of the present invention includes, between a pair of electrodes (anode and cathode), an EL layer which includes a light-emitting layer. The light-emitting layer has a stacked-layer structure including a first light-emitting layer and a second light-emitting layer. The first light-emitting layer is formed on the anode side and contains at least a first light-emitting substance (guest material) converting triplet excitation energy into light emission, a first organic compound (host material) having an electron-transport property, and a second organic compound (assist material) having a hole-transport property. The second light-emitting layer contains at least a second light-emitting substance (guest material) converting triplet excitation energy into light emission, a third organic compound (host material) having an electron-transport property, and the second organic compound (assist material) having a hole-transport property.

Note that the lowest unoccupied molecular orbital level (LUMO level) of the first organic compound (host material) contained in the first light-emitting layer is higher than that of the third organic compound (host material) contained in the second light-emitting layer. Therefore, excitation energy (E_(A)) of an exciplex formed in the first light-emitting layer can be designed to be higher than excitation energy (E_(B)) of an exciplex formed in the second light-emitting layer.

Further, the first light-emitting substance contained in the first light-emitting layer is a substance which emits light with a wavelength shorter than that of light emitted from the second light-emitting substance contained in the second light-emitting layer.

First, an element structure of a light-emitting element which is an example of the present invention is described with reference to FIG. 1A.

In the element structure illustrated in FIG. 1A, an EL layer 103 including a light-emitting layer 106 is provided between a pair of electrodes (an anode 101 and a cathode 102), and the EL layer 103 has a structure in which a hole-injection layer 104, a hole-transport layer 105, the light-emitting layer 106 (106 a and 106 b), an electron-transport layer 107, an electron-injection layer 108, and the like are sequentially stacked over the anode 101.

As illustrated in FIG. 1A, the light-emitting layer 106 in one embodiment of the present invention has a stacked-layer structure including the first light-emitting layer 106 a and the second light-emitting layer 106 b. The first light-emitting layer 106 a is formed on the anode side and contains at least a first light-emitting substance (guest material) 109 a converting triplet excitation energy into light emission, a first organic compound (host material) 110 having an electron-transport property, and a second organic compound (assist material) 111 having a hole-transport property. The second light-emitting layer 106 b contains at least a second light-emitting substance (guest material) 109 b converting triplet excitation energy into light emission, a third organic compound (host material) 112 having an electron-transport property, and the second organic compound (assist material) 111 having a hole-transport property. Electron-transport materials having an electron mobility of 10⁻⁶ cm²/Vs or more are mainly used as the first organic compound 110 and the third organic compound 112, and a hole-transport material having a hole mobility of 10⁻⁶ cm²/Vs or more is mainly used as the second organic compound 111. In this specification, the first organic compound 110 and the third organic compound 112 are referred to as host materials, and the second organic compound 111 is referred to as an assist material.

A combination of the first organic compound (host material) 110 and the second organic compound (assist material) 111 in the first light-emitting layer 106 a forms an exciplex (also referred to as an excited complex). Further, the emission wavelength of the exciplex formed by the first organic compound (host material) 110 and the second organic compound (assist material) 111 is located on the longer wavelength side with respect to the emission wavelength (fluorescent wavelength) of each of the first and second organic compounds (host and assist materials) 110 and 111. Therefore, the fluorescence spectrum of the first organic compound (host material) 110 and the fluorescence spectrum of the second organic compound (assist material) 111 can be converted into an emission spectrum which is located on the longer wavelength side.

The same applies to the second light-emitting layer 106 b. Thus, a combination of the third organic compound (host material) 112 and the second organic compound (assist material) 111 in the second light-emitting layer 106 b forms an exciplex (also referred to as an excited complex). Further, the emission wavelength of the exciplex formed by the third organic compound (host material) 112 and the second organic compound (assist material) 111 is located on the longer wavelength side with respect to the emission wavelength (fluorescent wavelength) of each of the third and second organic compounds (host and assist materials) 112 and 111. Therefore, the fluorescence spectrum of the third organic compound (host material) 112 and the fluorescence spectrum of the second organic compound (assist material) 111 can be converted into an emission spectrum which is located on the longer wavelength side.

Note that in the above structure, it is preferable that the level of a triplet excitation energy (T₁ level) of each of the first and second organic compounds (host and assist materials) 110 and 111 be higher than the T₁ level of the first light-emitting substance (guest material) 109 a converting triplet excitation energy into light emission. This is because, when the T₁ level of the first organic compound 110 (or the second organic compound 111) is lower than the T₁ level of the first light-emitting substance (guest material) 109 a, the triplet excitation energy of the first light-emitting substance (guest material) 109 a, which contributes to light emission, is quenched by the first organic compound 110 (or the second organic compound 111) and accordingly the emission efficiency decreases.

In a similar manner, it is preferable that the level of a triplet excitation energy (T₁ level) of each of the third and second organic compounds (host and assist materials) 112 and 111 be higher than the T₁ level of the second light-emitting substance (guest material) 109 b converting triplet excitation energy into light emission. This is because, when the T₁ level of the third organic compound 112 (or the second organic compound 111) is lower than the T₁ level of the second light-emitting substance (guest material) 109 b, the triplet excitation energy of the second light-emitting substance (guest material) 109 b, which contributes to light emission, is quenched by the third organic compound 112 (or the second organic compound 111) and accordingly the emission efficiency decreases.

Furthermore, in the first light-emitting layer 106 a included in the light-emitting layer 106, either the first organic compound (host material) 110 or the second organic compound (assist material) 111 may be contained in a higher proportion, and in the second light-emitting layer 106 b included in the light-emitting layer 106, either the third organic compound (host material) 112 or the second organic compound (assist material) 111 may be contained in a higher proportion; the present invention includes all the cases in its scope.

FIG. 1B is a band diagram which explains energy relation between the first organic compound (host material) 110, the second organic compound (assist material) 111, and the third organic compound (host material) 112 in the light-emitting layer 106 (the first light-emitting layer 106 a and the second light-emitting layer 106 b) having the above-described structure.

As illustrated in FIG. 1B, in the first light-emitting layer 106 a, the excitation energy (E_(A)) of the exciplex formed by the first organic compound (host material) 110 and the second organic compound (assist material) 111 is dependent on the HOMO level of the second organic compound (assist material) 111 and the LUMO level of the first organic compound (host material) 110. Further, in the second light-emitting layer 106 b, the excitation energy (E_(B)) of the exciplex formed by the third organic compound (host material) 112 and the second organic compound (assist material) 111 is dependent on the HOMO level of the second organic compound (assist material) 111 and the LUMO level of the third organic compound (host material) 112. Note that the first organic compound (host material) 110 contained in the first light-emitting layer 106 a has a higher LUMO level than the third organic compound (host material) 112 contained in the second light-emitting layer 106 b, which means that the excitation energy (E_(A)) of the exciplex formed in the first light-emitting layer 106 a can be designed to be higher than the excitation energy (E_(B)) of the exciplex formed in the second light-emitting layer 106 b.

Note that owing to the above-described element structure, the exciplex formed in the first light-emitting layer 106 a has higher excitation energy than the exciplex formed in the second light-emitting layer 106 b; therefore, when a substance emitting light with a wavelength shorter than that of light emitted from the second light-emitting substance (guest material) 109 b which is contained in the second light-emitting layer 106 b and converts triplet excitation energy into light emission is used as the first light-emitting substance (guest material) 109 a which is contained in the first light-emitting layer 106 a and converts triplet excitation energy into light emission, the first light-emitting layer 106 a and the second light-emitting layer 106 b can provide light emission at the same time. Further, part of excitation energy of the exciplex formed in the first light-emitting layer 106 a which does not contribute to light emission can be utilized as excitation energy for the second light-emitting substance (guest material) 109 b which converts triplet excitation energy into light emission in the second light-emitting layer 106 b. Accordingly, emission efficiency in the light-emitting element can be further increased.

In the light-emitting element described in this embodiment, exciplexes are formed in the light-emitting layer 106 (the first light-emitting layer 106 a and the second light-emitting layer 106 b), and the fluorescence spectrum of the first organic compound (host material) 110, the fluorescence spectrum of the third organic compound (host material) 112, or the fluorescence spectrum of the second organic compound (assist material) 111 can be converted into an emission spectrum which is located on the longer wavelength side. This means that even when the fluorescence spectrum of the first organic compound 110, the second organic compound 111, or the third organic compound 112 is located on the shorter wavelength side with respect to the absorption band on the longest wavelength side of the light-emitting substance 109 (the first light-emitting substance (guest material) 109 a and the second light-emitting substance (guest material) 109 b) converting triplet excitation energy into light emission as illustrated in FIG. 2, or does not have an overlap with the absorption band on the longest wavelength side of the light-emitting substance 109 (the first light-emitting substance (guest material) 109 a and the second light-emitting substance (guest material) 109 b) converting triplet excitation energy into light emission, the emission spectrum of the exciplex and the absorption band can have an increased overlap. Accordingly, the energy transfer efficiency in Formula (2-2B) above can be increased.

Furthermore, the exciplex is considered to have an extremely small difference between singlet excited energy and triplet excited energy. In other words, the emission spectrum of the exciplex from the singlet state and the emission spectrum thereof from the triplet state are highly close to each other. Accordingly, in the case where design is implemented such that the emission spectrum of the exciplex (generally the emission spectrum of the exciplex from the singlet state) overlaps with the absorption band on the longest wavelength side of the light-emitting substance 109 converting triplet excitation energy into light emission as described above, the emission spectrum of the exciplex from the triplet state (which is not observed at room temperature and also not observed in many cases at low temperature) also overlaps with the absorption band on the longest wavelength side of the light-emitting substance 109 converting triplet excitation energy into light emission. In other words, not only the efficiency of the energy transfer from the singlet excited state ((2-2)) but also the efficiency of the energy transfer from the triplet excited state ((2-1)) can be increased, and as a result, energy from both the singlet and triplet excited states can be efficiently converted into light emission.

Thus, molecular orbital calculations were performed as described below to verify whether or not an exciplex actually has such characteristics. In general, a combination of a heteroaromatic compound and an aromatic amine often forms an exciplex under the influence of the lowest unoccupied molecular orbital (LUMO) level of the heteroaromatic compound which is deeper than the LUMO level of the aromatic amine (the property of easily accepting electrons) and the highest occupied molecular orbital (HOMO) level of the aromatic amine which is shallower than the HOMO level of the heteroaromatic compound (the property of easily accepting holes). Thus, calculations were performed using a combination of dibenzo[f,h]quinoxaline (abbreviation: DBq), which is a typical skeleton forming the LUMO level of a heteroaromatic compound and is a model of the first organic compound 110 (or the third organic compound 112) in one embodiment of the present invention, and triphenylamine (abbreviation: TPA), which is a typical skeleton forming the HOMO level of an aromatic amine and is a model of the second organic compound 111 in one embodiment of the present invention.

First, the optimal molecular structures and the excitation energies of one molecule of DBq (abbreviation) and one molecule of TPA (abbreviation) in the lowest singlet excited state (S₁) and the lowest triplet excited state (T₁) were calculated using the time-dependent density functional theory (TD-DFT). Furthermore, the excitation energy of a dimer of DBq (abbreviation) and TPA (abbreviation) was also calculated.

In the DFT (density functional theory), the total energy is represented as the sum of potential energy, electrostatic energy between electrons, electronic kinetic energy, and exchange-correlation energy including all the complicated interactions between electrons. Also in the DFT, an exchange-correlation interaction is approximated by a functional (a function of another function) of one electron potential represented in terms of electron density to enable high-speed and high-accuracy calculations. Here, B3LYP which was a hybrid functional was used to specify the weight of each parameter related to exchange-correlation energy.

In addition, as a basis function, 6-311 (a basis function of a triple-split valence basis set using three contraction functions for each valence orbital) was applied to all the atoms.

By the above basis function, for example, is to 3s orbitals are considered in the case of hydrogen atoms, while 1s to 4s and 2p to 4p orbitals are considered in the case of carbon atoms. Furthermore, to improve calculation accuracy, the p function and the d function as polarization basis sets were added to hydrogen atoms and atoms other than hydrogen atoms, respectively.

Note that Gaussian 09 was used as a quantum chemistry computational program. A high performance computer (Altix 4700, manufactured by SGI Japan, Ltd.) was used for the calculations.

First, the HOMO levels and the LUMO levels of one molecule of DBq (abbreviation), one molecule of TPA (abbreviation), and a dimer of DBq (abbreviation) and TPA (abbreviation) were calculated. FIG. 3 shows the HOMO levels and the LUMO levels, and FIGS. 4A1, 4A2, 4B1, 4B2, 4C1, and 4C2 show HOMO and LUMO distributions.

FIG. 4A1 shows the LUMO distribution of one molecule of DBq (abbreviation); FIG. 4A2, the HOMO distribution of one molecule of DBq (abbreviation); FIG. 4B1, the LUMO distribution of one molecule of TPA (abbreviation); FIG. 4B2, the HOMO distribution of one molecule of TPA (abbreviation); FIG. 4C1, the LUMO distribution of the dimer of DBq (abbreviation) and TPA (abbreviation); and FIG. 4C2, the HOMO distribution of the dimer of DBq (abbreviation) and TPA (abbreviation).

As shown in FIG. 3, it is suggested that the dimer of DBq (abbreviation) and TPA (abbreviation) forms an exciplex of DBq (abbreviation) and TPA (abbreviation) under the influence of the LUMO level (−1.99 eV) of DBq (abbreviation) which is deeper (lower) than the LUMO level of TPA (abbreviation) and the HOMO level (−5.21 eV) of TPA (abbreviation) which is shallower (higher) than the HOMO level of DBq (abbreviation). In fact, as is clear from FIGS. 4C1 and 4C2, the LUMO of the dimer of DBq (abbreviation) and TPA (abbreviation) is distributed on the DBq (abbreviation) side, and the HOMO thereof is distributed on the TPA (abbreviation) side.

Next, excitation energies obtained from the optimal molecular structures of one molecule of DBq (abbreviation) at S₁ and T₁ levels will be shown. Here, the excitation energies at the S₁ and T₁ levels correspond to fluorescence and phosphorescence wavelengths, respectively, obtained from one molecule of DBq (abbreviation). The excitation energy at the S₁ level of one molecule of DBq (abbreviation) is 3.294 eV, and the fluorescence wavelength is 376.4 nm The excitation energy at the T₁ level of one molecule of DBq (abbreviation) is 2.460 eV, and the phosphorescence wavelength is 504.1 nm.

In addition, excitation energies obtained from the optimal molecular structures of one molecule of TPA (abbreviation) at S₁ and T₁ levels will be shown. Here, the excitation energies at the S₁ and T₁ levels correspond to fluorescence and phosphorescence wavelengths, respectively, obtained from one molecule of TPA (abbreviation). The excitation energy at the S₁ level of one molecule of TPA (abbreviation) is 3.508 eV, and the fluorescence wavelength is 353.4 nm The excitation energy at the T₁ level of one molecule of TPA (abbreviation) is 2.610 eV, and the phosphorescence wavelength is 474.7 nm.

Furthermore, excitation energies obtained from the optimal molecular structures of the dimer of DBq (abbreviation) and TPA (abbreviation) at S₁ and T₁ levels will be shown. The excitation energies at the S₁ and T₁ levels correspond to fluorescence and phosphorescence wavelengths, respectively, obtained from the dimer of DBq (abbreviation) and TPA (abbreviation). The excitation energy at the S₁ level of the dimer of DBq (abbreviation) and TPA (abbreviation) is 2.036 eV, and the fluorescence wavelength is 609.1 nm The excitation energy at the T₁ level of the dimer of DBq (abbreviation) and TPA (abbreviation) is 2.030 eV, and the phosphorescence wavelength is 610.0 nm.

It is found from the above that each of the phosphorescence wavelengths of one molecule of DBq (abbreviation) and one molecule of TPA (abbreviation) is shifted to the longer wavelength side by about 100 nm. This result shows a tendency similar to that of CBP (abbreviation) (measured values) described above and supports the validity of the calculations.

On the other hand, it is found that the fluorescence wavelength of the dimer of DBq (abbreviation) and TPA (abbreviation) is located on the longer wavelength side with respect to the fluorescence wavelengths of one molecule of DBq (abbreviation) and one molecule of TPA (abbreviation). It is also found that the difference between the fluorescence wavelength and the phosphorescence wavelength of the dimer of DBq (abbreviation) and TPA (abbreviation) is only 0.9 nm and that these wavelengths are substantially the same.

These results indicate that the singlet excitation energy and the triplet excitation energy of the exciplex are substantially equivalent. Therefore, it is indicated as described above that the exciplex can efficiently transfer energy from both the singlet state and the triplet state to the light-emitting substance (the guest materials including the first light-emitting substance and the second light-emitting substance) which converts triplet excitation energy into light emission.

In the above manner, the light-emitting element in one embodiment of the present invention transfers energy by utilizing an overlap between the emission spectrum of the exciplex formed in the light-emitting layer and the absorption spectrum of the light-emitting substance (the guest materials including the first light-emitting substance and the second light-emitting substance) which converts triplet excitation energy into light emission and thus has high energy transfer efficiency. Therefore, the light-emitting element can achieve high external quantum efficiency.

In addition, the exciplex exists only in an excited state and thus has no ground state capable of absorbing energy. Therefore, a phenomenon in which the light-emitting substance (guest material) converting triplet excitation energy into light emission is deactivated by energy transfer from the light-emitting substance (guest material) converting triplet excitation energy into light emission in the singlet excited state and triplet excited state to the exciplex before light emission (i.e., emission efficiency is lowered) is not considered to occur in principle. This also contributes to improvement of external quantum efficiency.

Note that the above-described exciplex is formed by an interaction between dissimilar molecules in excited states. The exciplex is generally known to be easily formed between a material having a relatively deep LUMO level and a material having a relatively shallow HOMO level.

An emission wavelength of the exciplex depends on a difference in energy between the HOMO level and the LUMO level. As a general tendency, when the energy difference is large, the emission wavelength is short, and when the energy difference is small, the emission wavelength is long.

Therefore, the HOMO levels and LUMO levels of the first organic compound (host material) 110, the second organic compound (assist material) 111, and the third organic compound (host material) 112 in this embodiment are different from each other as illustrated in FIG. 1B. Specifically, the energy levels vary in the following order: the HOMO levels of the first organic compound 110 and the third organic compound 112<the HOMO level of the second organic compound 111<the LUMO level of the third organic compound 112<the LUMO level of the first organic compound 110<the LUMO level of the second organic compound 111.

In each of the light-emitting layers, when the exciplex is formed by the two organic compounds (the first organic compound 110 and the second organic compound III in the first light-emitting layer 106 a, and the third organic compound 112 and the second organic compound 111 in the second light-emitting layer 106 b), the HOMO levels of the exciplexes in the first light-emitting layer 106 a and the second light-emitting layer 106 b depend on the second organic compound (assist material) 111, the LUMO level of the exciplex in the first light-emitting layer 106 a depends on the first organic compound (host material) 110, and the LUMO level of the exciplex in the second light-emitting layer 106 b depends on the LUMO level of the third organic compound (host material) 112. Therefore, the excitation energy (E_(A)) of the exciplex in the first light-emitting layer 106 a is higher than the excitation energy (E_(B)) of the exciplex in the second light-emitting layer 106 b. In other words, when a substance emitting light with a wavelength shorter than that of light emitted from the second light-emitting substance 109 b which is contained in the second light-emitting layer and converts triplet excitation energy into light emission is used as the first light-emitting substance 109 a which is contained in the first light-emitting layer and converts triplet excitation energy into light emission, a light-emitting element with high emission efficiency can be formed. Moreover, light-emitting materials with different emission wavelengths can efficiently emit light at the same time.

Note that the process of the exciplex formation in one embodiment of the present invention can be either of the following two processes.

One formation process is that an exciplex is formed from the second organic compound (assist material) having carriers (specifically, cation).

In general, when an electron and a hole recombine in a host material, excitation energy is transferred from the host material in an excited state to a guest material, whereby the guest material is brought into an excited state to emit light. Before the excitation energy is transferred from the host material to the guest material, the host material itself emits light or the excitation energy turns into thermal energy, which leads to partial deactivation of the excitation energy. In particular, when the host material is in a singlet excited state, energy transfer does not readily occur as described in (2-2). Such deactivation of excitation energy is one of causes for a decrease in lifetime of a light-emitting element.

However, in one embodiment of the present invention, an exciplex is formed from the first organic compound (or the third organic compound) (host material) and the second organic compound (assist material) having carriers (cation or anion); therefore, formation of a singlet exciton of the first organic compound (or the third organic compound) (host material) can be suppressed. In other words, there can be a process where an exciplex is directly formed without formation of a singlet exciton. Thus, deactivation of the singlet excitation energy can be inhibited. Accordingly, a light-emitting element having a long lifetime can be obtained.

For example, in the case where the first organic compound (or the third organic compound) is an electron-trapping compound having the property of easily capturing electrons (carrier) (having a deep LUMO level) among electron-transport materials and the second organic compound is a hole-trapping compound having the property of easily capturing holes (carrier) (having a shallow HOMO level) among hole-transport materials, an exciplex is formed directly from an anion of the first organic compound (or the third organic compound) and a cation of the second organic compound. An exciplex formed through such a process is particularly referred to as an electroplex. A light-emitting element having high emission efficiency can be obtained by suppressing the generation of the singlet excited state of the first organic compound (or the third organic compound) (host material) and transferring energy from an electroplex to the light-emitting substance (guest material) which converts triplet excitation energy into light emission, in the above-described manner. Note that in this case, the generation of the triplet excited state of the first organic compound (or the third organic compound) (host material) is similarly suppressed and an exciplex is directly formed; therefore, energy transfer is considered to occur from the exciplex to the light-emitting substance (guest material) which converts triplet excitation energy into light emission.

The other formation process is an elementary process where one of the host material (the first organic compound or the third organic compound) and the assist material (the second organic compound) forms a singlet exciton and then interacts with the other in the ground state to form an exciplex. Unlike an electroplex, a singlet excited state of the first organic compound (host material), the second organic compound (assist material), or the third organic compound (host material) is temporarily generated in this case, but this is rapidly converted into an exciplex, and thus, deactivation of singlet excitation energy can be inhibited. Thus, it is possible to inhibit deactivation of excitation energy of the first organic compound (host material), the second organic compound (assist material), or the third organic compound (host material). Note that in this case, it is considered that the triplet excited state of the host material is similarly rapidly converted into an exciplex and energy is transferred from the exciplex to the light-emitting substance (guest material) which converts triplet excitation energy into light emission.

Note that in the case where the first organic compound (or the third organic compound) (host material) is an electron-trapping compound, the second organic compound (assist material) is a hole-trapping compound, and the difference between the HOMO levels and the difference between the LUMO levels of these compounds are large (specifically, 0.3 eV or more), electrons are selectively injected into the first organic compound (or the third organic compound) (host material) and holes are selectively injected into the second organic compound (assist material). In this case, it is thought that the process where an electroplex is formed takes precedence over the process where an exciplex is formed through a singlet exciton.

To make the emission spectrum of the exciplex and the absorption spectrum of the light-emitting substance (guest material) converting triplet excitation energy into light emission sufficiently overlap each other, the difference between the energy of a peak of the emission spectrum and the energy of a peak of the absorption band on the lowest energy side in the absorption spectrum is preferably 0.3 eV or less. The difference is more preferably 0.2 eV or less, even more preferably 0.1 eV or less.

In the light-emitting element in one embodiment of the present invention, it is also preferable that the excitation energy of the exciplex be sufficiently transferred to the light-emitting substance (guest material) which converts triplet excitation energy into light emission, and that light emission from the exciplex be not substantially observed. Therefore, it is preferable that energy be transferred to the light-emitting substance which converts triplet excitation energy into light emission through an exciplex so that the light-emitting substance which converts triplet excitation energy into light emission emits light. Note that as the light-emitting substance which converts triplet excitation energy into light emission, a phosphorescent compound (e.g., an organometallic complex), a thermally activated delayed fluorescence (TADF) material, or the like is preferably used.

In the case where a light-emitting substance which converts triplet excitation energy into light emission is used as the first organic compound in the first light emitting layer 106 a (the third organic compound in the case of the second light-emitting layer 106 b) (host material) of the light-emitting element in one embodiment of the present invention, the first organic compound (the third organic compound in the case of the second light-emitting layer 106 b) (host material) itself readily emits light and does not readily allow energy to be transferred to the light-emitting substance (guest material) which converts triplet excitation energy into light emission. In this case, it is favorable if the first organic compound (the third organic compound in the case of the second light-emitting layer 106 b) could emit light efficiently, but it is difficult to achieve high emission efficiency because a high concentration of the host material may cause a problem of concentration quenching. Therefore, the case where at least one of the first organic compound (the third organic compound in the case of the second light-emitting layer 106 b) (host material) and the second organic compound (assist material) is a fluorescent compound (i.e., a compound which readily undergoes light emission or thermal deactivation from the singlet excited state) is effective. For this reason, it is preferable that at least one of the first organic compound (the third organic compound in the case of the second light-emitting layer 106 b) (host material) and the second organic compound (assist material) be a fluorescent compound and an exciplex be used as a medium for energy transfer.

Note that the structure described in this embodiment can be combined as appropriate with any of the structures described in the other embodiments.

(Embodiment 2)

In this embodiment, an example of a light-emitting element in one embodiment of the present invention is described with reference to FIG. 5.

In the light-emitting element described in this embodiment, as illustrated in FIG. 5, an EL layer 203 including a light-emitting layer 206 is provided between a pair of electrodes (a first electrode (anode) 201 and a second electrode (cathode) 202), and the EL layer 203 includes a hole-injection layer 204, a hole-transport layer 205, an electron-transport layer 207, an electron-injection layer 208, and the like in addition to the light-emitting layer 206 having a stacked-layer structure including a first light-emitting layer 206 a and a second light-emitting layer 206 b.

Note that the light-emitting layer 206 in this embodiment has a stacked-layer structure including the first light-emitting layer 206 a and the second light-emitting layer 206 b. In the light-emitting layer 206, the first light-emitting layer 206 a contains a first light-emitting substance (guest material) 209 a converting triplet excitation energy into light emission, a first organic compound (host material) 210 having an electron-transport property, and a second organic compound (assist material) 211 having a hole-transport property; and the second light-emitting layer 206 b contains a second light-emitting substance (guest material) 209 b converting triplet excitation energy into light emission, a third organic compound (host material) 212 having an electron-transport property, and the second organic compound (assist material) 211 having a hole-transport property.

Note that the lowest unoccupied molecular orbital level (LUMO level) of the first organic compound (host material) contained in the first light-emitting layer is higher than that of the third organic compound (host material) contained in the second light-emitting layer. Therefore, excitation energy (E_(A)) of an exciplex formed in the first light-emitting layer can be designed to be higher than excitation energy (E_(B)) of an exciplex formed in the second light-emitting layer.

Note that crystallization of the light-emitting layer 206 (the first light-emitting layer 206 a and the second light-emitting layer 206 b) can be suppressed by employing a structure in which the first light-emitting substance 209 a converting triplet excitation energy into light emission is dispersed in the first organic compound (host material) 210 and the second organic compound (assist material) 211 in the first light-emitting layer 206 a, and the second light-emitting substance 209 b converting triplet excitation energy into light emission is dispersed in the third organic compound (host material) 212 and the second organic compound (assist material) 211 in the second light-emitting layer 206 b. Further, it is possible to suppress concentration quenching due to high concentration of the light-emitting substance 209 (209 a and 209 b), and thus the light-emitting element can have higher emission efficiency.

It is preferable that the level of a triplet excitation energy (T₁ level) of each of the first organic compound 210, the second organic compound 211, and the third organic compound 212 be higher than the T₁ level of the light-emitting substance 209 (209 a and 209 b) converting triplet excitation energy into light emission. This is because, when the T₁ level of each of the first organic compound 210, the second organic compound 211, and the third organic compound 212 is lower than the T₁ level of the light-emitting substance 209 (209 a and 209 b) converting triplet excitation energy into light emission, the triplet excitation energy of the light-emitting substance 209 (209 a and 209 b) converting triplet excitation energy into light emission, which contributes to light emission, is quenched by the first organic compound 210 (or the second organic compound 211, or the third organic compound 212) and accordingly the emission efficiency decreases.

In the light-emitting layer 206 in this embodiment, at the time of recombination of carriers (electrons and holes) injected from the respective electrodes, the first organic compound 210 and the second organic compound 211 form an exciplex in the first light-emitting layer 206 a, and the third organic compound 212 and the second organic compound 211 form an exciplex in the second light-emitting layer 206 b. Thus, a fluorescence spectrum of the first organic compound 210 and that of the second organic compound 211 in the first light-emitting layer 206 a can be converted into an emission spectrum of the exciplex which is located on a longer wavelength side, and a fluorescence spectrum of the third organic compound 212 and that of the second organic compound 211 in the second light-emitting layer 206 b can be converted into an emission spectrum of the exciplex which is located on a longer wavelength side. In consideration of this, the first organic compound 210 and the second organic compound 211 in the first light-emitting layer 206 a and the third organic compound 212 and the second organic compound 211 in the second light-emitting layer 206 b are selected to make an overlap between the emission spectrum of the exciplex and the absorption spectrum of the light-emitting substance (guest material) 209 converting triplet excitation energy into light emission large and thereby to maximize energy transfer from a singlet excited state. That is, it is assumed here that energy transfer from the exciplex, not the host material, occurs also in the case of a triplet excited state.

Note that as the light-emitting substance 209 (the first light-emitting substance 209 a and the second light-emitting substance 209 b) which converts triplet excitation energy into light emission, a phosphorescent compound (e.g., an organometallic complex), a thermally activated delayed fluorescence (TADF) material, or the like is preferably used. Electron-transport materials are preferably used as the first organic compound (host material) 210 and the third organic compound (host material) 212. A hole-transport material is preferably used as the second organic compound (assist material) 211.

Note that examples of the organometallic complex include bis[2-(4′,6′-difluorophenyl)pyridinato-N,C²′]iridium(III) tetrakis(1-pyrazolyl)borate (abbreviation: FIr6), bis[2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)]iridium(III) picolinate (abbreviation: FIrpic), bis[2-(3′,5′-bistrifluoromethylphenyl)pyridinato-N,C²′]iridium(III) picolinate (abbreviation: Ir(CF₃ppy)₂(pic)), bis[2-(4′,6′-difluorophenyl)pyridinato-N, C²′]iridium(III) acetylacetonate (abbreviation: FIracac), tris(2-phenylpyridinato)iridium(III) (abbreviation: Ir(ppy)₃), bis(2-phenylpyridinato)iridium(III) acetylacetonate (abbreviation: Ir(ppy)₂(acac)), bis(benzo[h]quinolinato)iridium(III) acetylacetonate (abbreviation: Ir(bzq)₂(acac)), bis(2,4-diphenyl-1,3-oxazolato-N,C²′)iridium(III) acetylacetonate (abbreviation: Ir(dpo)₂(acac)), bis{2-[4′-(perfluorophenyl)phenyl]pyridinato-N,C^(2′)}iridium(III) acetylacetonate (abbreviation: Ir(p-PF-ph)₂(acac)), bis(2-phenylbenzothiazolato-N,C²′)iridium(III) acetylacetonate (abbreviation: Ir(bt)₂(acac)), bis[2-(2′-benzo[4,5-α]thienyl)pyridinato-N,C³′]iridium(III) acetylacetonate (abbreviation: Ir(btp)₂(acac)), bis(1-phenylisoquinolinato-N,C²′)iridium(III) acetylacetonate (abbreviation: Ir(piq)₂(acac)), (acetylacetonato)bis[2,3-bis(4-fluorophenyl)quinoxalinato]iridium(III) (abbreviation: Ir(Fdpq)₂(acac)), (acetylacetonato)bis(2,3,5-triphenylpyrazinato)iridium(III) (abbreviation: Ir(tppr)₂(acac)), 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin platinum(II) (abbreviation: PtOEP), tris(acetylacetonato)(monophenanthroline)terbium(III) (abbreviation: Tb(acac)₃(Phen)), tris(1,3-diphenyl-1,3-propanedionato)(monophenanthroline)europium(III) (abbreviation: Eu(DBM)₃(Phen)), and tris[1-(2-thenoyl)-3,3,3-trifluoroacetonato](monophenanthro line)europium(III) (abbreviation: Eu(TTA)₃(Phen)).

As the electron-transport material, a π-electron deficient heteroaromatic compound such as a nitrogen-containing heteroaromatic compound is preferable, examples of which include quinoxaline derivatives and dibenzoquinoxaline derivatives such as 2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation: 2mDBTPDBq-II), 2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline (abbreviation: 2mDBTBPDBq-II), 2-[4-(3,6-diphenyl-9H-carbazol-9-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation: 2CzPDBq-III), 7-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation: 7mDBTPDBq-II), and 6-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation: 6mDBTPDBq-II).

As the hole-transport material, a π-electron rich heteroaromatic compound (e.g., a carbazole derivative or an indole derivative) or an aromatic amine compound is preferable, examples of which include 4-phenyl-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation: PCBA1BP), 4,4′-di(1-naphthyl)-4″-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation: PCBNBB), 3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)-amino]-9-phenylcarbazole (abbreviation: PCzPCN1), 4,4′,4″-tris[N-(1-naphthyl)-N-phenylamino]triphenylamine (abbreviation: 1′-TNATA), 2,7-bis[N-(4-diphenylaminophenyl)-N-phenylamino]-spiro-9,9′-bifluorene (abbreviation: DPA2 SF), N,N′-bis(9-phenylcarbazol-3-yl)-N,N′-diphenylbenzene-1,3-diamine (abbreviation: PCA2B), N-(9,9-dimethyl-2-diphenylamino-9H-fluoren-7-yl)diphenylamine (abbreviation: DPNF), N,N′,N″-triphenyl-N,N′,N″-tris(9-phenylcarbazol-3-yl)benzene-1,3,5-triamine (abbreviation: PCA3B), 2-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]spiro-9,9′-bifluorene (abbreviation: PCASF), 2-[N-(4-diphenylaminophenyl)-N-phenylamino]spiro-9,9′-bifluorene (abbreviation: DPASF), N,N′-bis[4-(carbazol-9-yl)phenyl]-N,N′-diphenyl-9,9-dimethylfluorene-2,7-diamine (abbreviation: YGA2F), 4,4′-bis[N-(3-methylphenyl)-N-phenylamino]biphenyl (abbreviation: TPD), 4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl (abbreviation: DPAB), N-(9,9-dimethyl-9H-fluoren-2-yl)-N-{9,9-dimethyl-2-[Y-phenyl-N′-(9,9-dimethyl-9H-fluoren-2-yl)-amino]-9H-fluoren-7-yl}phenylamine (abbreviation: DFLADFL), 3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole (abbreviation: PCzPCA1), 3-[N-(4-diphenyl aminophenyl)-N-phenyl amino]-9-phenylcarbazole (abbreviation: PCzDPA1), 3,6-bis[N-(4-diphenylaminophenyl)-N-phenylamino]-9-phenylcarbazole (abbreviation: PCzDPA2), 4,4′-bis(N-{4-[N′-(3-methylphenyl)-N-phenylamino]phenyl}-N-phenylamino)biphenyl (abbreviation: DNTPD), 3,6-bis[N-(4-diphenylaminophenyl)-N-(1-naphthyl)amino]-9-phenylcarbazole (abbreviation: PCzTPN2), and 3,6-bis[N-(9-phenyl carbazol-3-yl)-N-phenylamino]-9-phenylcarbazole (abbreviation: PCzPCA2).

Note that materials which can be used for the light-emitting substance (guest material) 209 (the first light-emitting substance 209 a and the second light-emitting substance 209 b) converting triplet excitation energy into light emission, the first organic compound (host material) 210, the second organic compound (assist material) 211, and the third organic compound (host material) 212 are not limited to the above examples. The combination is determined so that an exciplex can be formed, the emission spectrum of the exciplex overlaps with the absorption spectrum of the light-emitting substance (guest material) 209 (the first light-emitting substance 209 a or the second light-emitting substance 209 b) converting triplet excitation energy into light emission, and the peak of the emission spectrum of the exciplex has a longer wavelength than the peak of the absorption spectrum of the light-emitting substance (guest material) 209 (the first light-emitting substance 209 a or the second light-emitting substance 209 b) converting triplet excitation energy into light emission.

In the case where an electron-transport material is used as the first organic compound 210 and a hole-transport material is used as the second organic compound 211, carrier balance can be controlled by the mixture ratio of the compounds. Specifically, the ratio of the first organic compound 210 to the second organic compound 211 is preferably 1:9 to 9:1.

A specific example in which the light-emitting element described in this embodiment is manufactured is described below.

For the first electrode (anode) 201 and the second electrode (cathode) 202, a metal, an alloy, an electrically conductive compound, a mixture thereof, or the like can be used. Specifically, indium oxide-tin oxide (ITO: indium tin oxide), indium oxide-tin oxide containing silicon or silicon oxide, indium oxide-zinc oxide (indium zinc oxide), indium oxide containing tungsten oxide and zinc oxide, gold (Au), platinum (Pt), nickel (Ni), tungsten (W), chromium (Cr), molybdenum (Mo), iron (Fe), cobalt (Co), copper (Cu), palladium (Pd), or titanium (Ti) can be used. In addition, an element belonging to Group 1 or Group 2 of the periodic table, for example, an alkali metal such as lithium (Li) or cesium (Cs), an alkaline earth metal such as magnesium (Mg), calcium (Ca), or strontium (Sr), an alloy containing such an element (e.g., MgAg or AlLi), a rare earth metal such as europium (Eu) or ytterbium (Yb), an alloy containing such an element, graphene, or the like can be used. The first electrode (anode) 201 and the second electrode (cathode) 202 can be formed by, for example, a sputtering method, an evaporation method (including a vacuum evaporation method), or the like.

Examples of a substance having a high hole-transport property which is used for the hole-injection layer 204 and the hole-transport layer 205 include aromatic amine compounds such as 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB or α-NPD), N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine (abbreviation: TPD), 4,4′,4″-tris(carbazol-9-yl)triphenylamine (abbreviation: TCTA), 4,4′,4″-tris(N,N-diphenylamino)triphenylamine (abbreviation: TDATA), 4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine (abbreviation: MTDATA), and 4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl (abbreviation: BSPB), 3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole (abbreviation: PCzPCA1), 3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole (abbreviation: PCzPCA2), and 3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)-amino]-9-phenylcarbazole (abbreviation: PCzPCN1). Other examples include carbazole derivatives such as 4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP), 1,3,5-tris[4-(N-carbazolyl)-phenyl]benzene (abbreviation: TCPB), and 9-[4-(10-phenyl-9-anthracenyl)phenyl]-9H-carbazole (abbreviation: CzPA). The substances mentioned here are mainly substances that have a hole mobility of 10⁻⁶ cm²/Vs or more. Note that other than these substances, any substance that has a property of transporting more holes than electrons may be used.

Still other examples include high molecular compounds such as poly(N-vinylcarbazole) (abbreviation: PVK), poly(4-vinyltriphenylamine) (abbreviation: PVTPA), poly[N-(4-{N′-[4-(4-diphenylamino)phenyl]phenyl-N′-phenylamino}phenyl)methacrylamide] (abbreviation: PTPDMA), and poly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine] (abbreviation: Poly-TPD).

Further, examples of an acceptor substance which can be used for the hole-injection layer 204 include oxides of transition metals, oxides of metals belonging to Groups 4 to 8 of the periodic table, and the like. Specifically, molybdenum oxide is particularly preferable.

The light-emitting layer 206 (206 a and 206 b) is formed such that, as described above, the first light-emitting layer 206 a contains at least the first light-emitting substance 209 a, the first organic compound (host material) 210, and the second organic compound (assist material) 211, and the second light-emitting layer 206 b contains at least the second light-emitting substance 209 b, the third organic compound (host material) 212, and the second organic compound (assist material) 211.

The electron-transport layer 207 is a layer that contains a substance having a high electron-transport property. For the electron-transport layer 207, it is possible to use a metal complex such as Alq₃, tris(4-methyl-8-quinolinolato)aluminum (abbreviation: Almq₃), bis(10-hydroxybenzo[h]quinolinato)beryllium (abbreviation: BeBq₂), BAlq, Zn(BOX)₂, or bis[2-(2-hydroxyphenyl)benzothiazolato]zinc (abbreviation: Zn(BTZ)₂). Alternatively, it is possible to use a heteroaromatic compound such as 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation: PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene (abbreviation: OXD-7), 3-(4-tert-butylphenyl)-4-phenyl-5-(4-biphenylyl)-1,2,4-triazole (abbreviation: TAZ), 3-(4-tert-butylphenyl)-4-(4-ethylphenyl)-5-(4-biphenylyl)-1,2,4-triazole (abbreviation: p-EtTAZ), bathophenanthroline (abbreviation: BPhen), bathocuproine (abbreviation: BCP), or 4,4′-bis(5-methylbenzoxazol-2-yl)stilbene (abbreviation: BzOs). Further alternatively, it is possible to use a high molecular compound such as poly(2,5-pyridinediyl) (abbreviation: PPy), poly[(9,9-dihexylfluorene-2,7-diyl)-co-(pyridine-3,5-diyl)] (abbreviation: PF-Py), or poly[(9,9-dioctylfluorene-2,7-diyl)-co-(2,2′-bipyridine-6,6′-diyl)] (abbreviation: PF-BPy). The substances mentioned here are mainly substances that have an electron mobility of 10⁻⁶ cm²/Vs or more. Note that other than these substances, any substance that has a property of transporting more electrons than holes may be used for the electron-transport layer 207.

The electron-transport layer 207 is not limited to a single layer, and may be a stack of two or more layers containing any of the above substances.

The electron-injection layer 208 is a layer that contains a substance having a high electron-injection property. Examples of the substance that can be used for the electron-injection layer 208 include alkali metals, alkaline earth metals, and compounds thereof, such as lithium fluoride (LiF), cesium fluoride (CsF), calcium fluoride (CaF₂), and lithium oxide (LiO_(x)), and rare earth metal compounds, such as erbium fluoride (ErF₃). Alternatively, the above-mentioned substances for forming the electron-transport layer 207 can be used.

Alternatively, a composite material in which an organic compound and an electron donor (a donor) are mixed may be used for the electron-injection layer 208. Such a composite material, in which electrons are generated in the organic compound by the electron donor, has high electron-injection and electron-transport properties. The organic compound here is preferably a material excellent in transporting the generated electrons, and specifically any of the above substances (such as metal complexes and heteroaromatic compounds) for the electron-transport layer 207 can be used. As the electron donor, a substance showing an electron-donating property with respect to the organic compound may be used. Specifically, alkali metals, alkaline earth metals, and rare earth metals are preferable, and lithium, cesium, magnesium, calcium, erbium, ytterbium, and the like can be given. Any of alkali metal oxides and alkaline earth metal oxides is preferable, examples of which are lithium oxide, calcium oxide, barium oxide, and the like, and a Lewis base such as magnesium oxide or an organic compound such as tetrathiafulvalene (abbreviation: TTF) can be used.

Note that the hole-injection layer 204, the hole-transport layer 205, the light-emitting layer 206 (206 a and 206 b), the electron-transport layer 207, and the electron-injection layer 208 which are mentioned above can each be formed by a method such as an evaporation method (including a vacuum evaporation method), an inkjet method, or a coating method.

Light emission obtained in the light-emitting layer 206 of the above-described light-emitting element is extracted to the outside through either the first electrode 201 or the second electrode 202 or both. Therefore, either the first electrode 201 or the second electrode 202 in this embodiment, or both, is an electrode having a light-transmitting property.

In the light-emitting element described in this embodiment, energy transfer efficiency can be improved owing to energy transfer utilizing an overlap between an emission spectrum of an exciplex and an absorption spectrum of a light-emitting substance (guest material) which converts triplet excitation energy into light emission; accordingly, the light-emitting element can achieve high external quantum efficiency.

Note that the light-emitting element described in this embodiment is one embodiment of the present invention and is particularly characterized by the structure of the light-emitting layer. Therefore, when the structure described in this embodiment is employed, a passive matrix light-emitting device, an active matrix light-emitting device, and the like can be manufactured. Each of these light-emitting devices is included in the present invention.

Note that there is no particular limitation on the structure of a TFT in the case of manufacturing the active matrix light-emitting device. For example, a staggered TFT or an inverted staggered TFT can be used as appropriate. Further, a driver circuit formed over a TFT substrate may be formed using both an n-type TFT and a p-type TFT or either an n-type TFT or a p-type TFT. Furthermore, there is also no particular limitation on the crystallinity of a semiconductor film used for the TFT. For example, an amorphous semiconductor film, a crystalline semiconductor film, an oxide semiconductor film, or the like can be used.

Note that the structure described in this embodiment can be combined as appropriate with any of the structures described in the other embodiments.

(Embodiment 3)

In this embodiment, as one embodiment of the present invention, a light-emitting element (hereinafter referred to as tandem light-emitting element) in which a charge generation layer is provided between a plurality of EL layers will be described.

The light-emitting element described in this embodiment is a tandem light-emitting element including a plurality of EL layers (a first EL layer 302(1) and a second EL layer 302(2)) between a pair of electrodes (a first electrode 301 and a second electrode 304) as illustrated in FIG. 6A.

In this embodiment, the first electrode 301 functions as an anode, and the second electrode 304 functions as a cathode. Note that the first electrode 301 and the second electrode 304 can have structures similar to those described in Embodiment 1. In addition, all or any of the plurality of EL layers (the first EL layer 302(1) and the second EL layer 302(2)) may have structures similar to those described in Embodiment 1 or 2. In other words, the structures of the first EL layer 302(1) and the second EL layer 302(2) may be the same or different from each other and can be similar to those of the EL layers described in Embodiment 1 or 2.

Further, a charge generation layer (I) 305 is provided between the plurality of EL layers (the first EL layer 302(1) and the second EL layer 302(2)). The charge generation layer (I) 305 has a function of injecting electrons into one of the EL layers and injecting holes into the other of the EL layers when a voltage is applied between the first electrode 301 and the second electrode 304. In this embodiment, when a voltage is applied such that the potential of the first electrode 301 is higher than that of the second electrode 304, the charge generation layer (I) 305 injects electrons into the first EL layer 302(1) and injects holes into the second EL layer 302(2).

Note that in terms of light extraction efficiency, the charge generation layer (I) 305 preferably has a light-transmitting property with respect to visible light (specifically, the charge generation layer (I) 305 preferably has a visible light transmittance of 40% or more). Further, the charge generation layer (I) 305 functions even if it has lower conductivity than the first electrode 301 or the second electrode 304.

The charge generation layer (I) 305 may have either a structure in which an electron acceptor (acceptor) is added to an organic compound having a high hole-transport property or a structure in which an electron donor (donor) is added to an organic compound having a high electron-transport property. Alternatively, both of these structures may be stacked.

In the case where the electron acceptor is added to the organic compound having a high hole-transport property, examples of the organic compound having a high hole-transport property include aromatic amine compounds such as NPB, TPD, TDATA, MTDATA, and 4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl (abbreviation: BSPB), and the like. The substances mentioned here are mainly substances that have a hole mobility of 10⁻⁶ cm²/Vs or more. Note that other than these substances, any organic compound that has a property of transporting more holes than electrons may be used.

Examples of the electron acceptor include 7,7,8,8-tetracyano-2,3,5,6-tetrafluoro quino dimethane (abbreviation: F₄-TCNQ), chloranil, oxides of transition metals, and oxides of metals that belong to Groups 4 to 8 of the periodic table. Specifically, vanadium oxide, niobium oxide, tantalum oxide, chromium oxide, molybdenum oxide, tungsten oxide, manganese oxide, and rhenium oxide are preferable because of their high electron-accepting property. Among these, molybdenum oxide is especially preferable since it is stable in the air, has a low hygroscopic property, and is easy to handle.

On the other hand, in the case where the electron donor is added to the organic compound having a high electron-transport property, examples of the organic compound having a high electron-transport property which can be used are metal complexes having a quinoline skeleton or a benzoquinoline skeleton, such as Alq, Almq₃, BeBc₁₂, and BAlq, and the like. Other examples are metal complexes having an oxazole-based or thiazole-based ligand, such as Zn(BOX)₂ and Zn(BTZ)₂. Other than metal complexes, PBD, OXD-7, TAZ, BPhen, BCP, or the like can be used. The substances mentioned here are mainly substances that have an electron mobility of 10⁻⁶ cm²/Vs or more. Note that other than these substances, any organic compound that has a property of transporting more electrons than holes may be used.

Examples of the electron donor which can be used are alkali metals, alkaline earth metals, rare earth metals, metals that belong to Group 13 of the periodic table, and oxides or carbonates thereof. Specifically, lithium (Li), cesium (Cs), magnesium (Mg), calcium (Ca), ytterbium (Yb), indium (In), lithium oxide, cesium carbonate, and the like are preferable. An organic compound, such as tetrathianaphthacene, may be used as the electron donor.

Note that forming the charge generation layer (I) 305 by using any of the above materials can suppress a drive voltage increase caused by the stack of the EL layers.

Although this embodiment shows the light-emitting element having two EL layers, the present invention can be similarly applied to a light-emitting element in which n EL layers (n is 3 or more) are stacked as illustrated in FIG. 6B. In the case where a plurality of EL layers is included between a pair of electrodes as in the light-emitting element according to this embodiment, by provision of the charge generation layers (I) between the EL layers, light emission in a high luminance region can be obtained with current density kept low. Since the current density can be kept low, the element can have a long lifetime. When the light-emitting element is applied to lighting, voltage drop due to resistance of an electrode material can be reduced, thereby achieving homogeneous light emission in a large area. Moreover, it is possible to achieve a light-emitting device which can be driven at a low voltage and has low power consumption.

Furthermore, by making emission colors of EL layers different, light of a desired color can be obtained from the light-emitting element as a whole. For example, the emission colors of first and second EL layers are complementary in a light-emitting element having the two EL layers, so that the light-emitting element can be made to emit white light as a whole. Note that the term “complementary” means color relationship in which an achromatic color is obtained when colors are mixed. That is, emission of white light can be obtained by mixture of light emitted from substances whose emission colors are complementary colors.

Further, the same applies to a light-emitting element having three EL layers. For example, the light-emitting element as a whole can emit white light when the emission color of the first EL layer is red, the emission color of the second EL layer is green, and the emission color of the third EL layer is blue.

Note that the structure described in this embodiment can be combined as appropriate with any of the structures described in the other embodiments.

(Embodiment 4)

In this embodiment, a light-emitting device which is one embodiment of the present invention will be described.

A light-emitting device described in this embodiment has a micro optical resonator (microcavity) structure in which a light resonant effect between a pair of electrodes is utilized. The light-emitting device includes a plurality of light-emitting elements each of which has at least an EL layer 405 between a pair of electrodes (a reflective electrode 401 and a semi-transmissive and semi-reflective electrode 402) as illustrated in FIG. 7. Further, the EL layer 405 includes at least light-emitting layers 404 (404R, 404G, and 404B) each serving as a light-emitting region and may further include a hole-injection layer, a hole-transport layer, an electron-transport layer, an electron-injection layer, a charge generation layer (E), and the like. Note that the light-emitting layers 404 (404R, 404G, and 404B) may have the structure of the light-emitting layer according to one embodiment of the present invention which is described in Embodiment 1 or 2.

In this embodiment, a light-emitting device is described which includes light-emitting elements (a first light-emitting element (R) 410R, a second light-emitting element (G) 410G, and a third light-emitting element (B) 410B) having different structures as illustrated in FIG. 7.

The first light-emitting element (R) 410R has a structure in which a first transparent conductive layer 403 a; an EL layer 405 including a first light-emitting layer (B) 404B, a second light-emitting layer (G) 404G, and a third light-emitting layer (R) 404R in part; and a semi-transmissive and semi-reflective electrode 402 are sequentially stacked over a reflective electrode 401. The second light-emitting element (G) 410G has a structure in which a second transparent conductive layer 403 b, the EL layer 405, and the semi-transmissive and semi-reflective electrode 402 are sequentially stacked over the reflective electrode 401. The third light-emitting element (B) 410B has a structure in which the EL layer 405 and the semi-transmissive and semi-reflective electrode 402 are sequentially stacked over the reflective electrode 401.

Note that the reflective electrode 401, the EL layer 405, and the semi-transmissive and semi-reflective electrode 402 are common to the light-emitting elements (the first light-emitting element (R) 410R, the second light-emitting element (G) 410G, and the third light-emitting element (B) 410B). The first light-emitting layer (B) 404B emits light (λ_(B)) having a peak in a wavelength region from 420 nm to 480 nm The second light-emitting layer (G) 404G emits light (λ_(G)) having a peak in a wavelength region from 500 nm to 550 nm The third light-emitting layer (R) 404R emits light (λ_(R)) having a peak in a wavelength region from 600 nm to 760 nm Thus, in each of the light-emitting elements (the first light-emitting element (R) 410R, the second light-emitting element (G) 4100 and the third light-emitting element (B) 410B), light emitted from the first light-emitting layer (B) 404B, light emitted from the second light-emitting layer (G) 404G, and light emitted from the third light-emitting layer (R) 404R overlap with each other; accordingly, light having a broad emission spectrum that covers a visible light region can be emitted. Note that the above wavelengths satisfy the relation of λ_(B)<λ_(G)<λ_(R).

Each of the light-emitting elements described in this embodiment has a structure in which the EL layer 405 is provided between the reflective electrode 401 and the semi-transmissive and semi-reflective electrode 402. Light emitted in all directions from the light-emitting layers included in the EL layer 405 is resonated by the reflective electrode 401 and the semi-transmissive and semi-reflective electrode 402 which function as a micro optical resonator (microcavity). Note that the reflective electrode 401 is formed using a conductive material having reflectivity, and a film whose visible light reflectivity is 40% to 100%, preferably 70% to 100%, and whose resistivity is 1×10⁻² Ωcm or lower is used. In addition, the semi-transmissive and semi-reflective electrode 402 is formed using a conductive material having reflectivity and a conductive material having a light-transmitting property, and a film whose visible light reflectivity is 20% to 80%, preferably 40% to 70%, and whose resistivity is 1×10⁻² Ωcm or lower is used.

In this embodiment, the thicknesses of the transparent conductive layers (the first transparent conductive layer 403 a and the second transparent conductive layer 403 b) provided in the first light-emitting element (R) 410R and the second light-emitting element (G) 410G, respectively, are varied between the light-emitting elements, whereby the light-emitting elements differ from each other in the optical path length from the reflective electrode 401 to the semi-transmissive and semi-reflective electrode 402. In other words, in light having a broad emission spectrum, which is emitted from the light-emitting layers of each of the light-emitting elements, light with a wavelength that is resonated between the reflective electrode 401 and the semi-transmissive and semi-reflective electrode 402 can be intensified while light with a wavelength that is not resonated therebetween can be attenuated. Thus, when the elements differ from each other in the optical path length from the reflective electrode 401 to the semi-transmissive and semi-reflective electrode 402, light with different wavelengths can be extracted.

Note that the optical path length (also referred to as optical distance) is expressed as a product of an actual distance and a refractive index, and in this embodiment, is a product of an actual thickness and n (refractive index). That is, an optical path length=actual thickness×n.

Further, the total thickness from the reflective electrode 401 to the semi-transmissive and semi-reflective electrode 402 is set to mλ_(R)/2 (m is a natural number) in the first light-emitting element (R) 410R; the total thickness from the reflective electrode 401 to the semi-transmissive and semi-reflective electrode 402 is set to mλ_(G)/2 (m is a natural number) in the second light-emitting element (G) 410G; and the total thickness from the reflective electrode 401 to the semi-transmissive and semi-reflective electrode 402 is set to mλ_(B)/2 (m is a natural number) in the third light-emitting element (B) 410B.

In this manner, the light (λ_(R)) emitted from the third light-emitting layer (R) 404R included in the EL layer 405 is mainly extracted from the first light-emitting element (R) 410R, the light (λ_(G)) emitted from the second light-emitting layer (G) 404G included in the EL layer 405 is mainly extracted from the second light-emitting element (G) 410G, and the light (λ_(B)) emitted from the first light-emitting layer (B) 404B included in the EL layer 405 is mainly extracted from the third light-emitting element (B) 410B. Note that the light extracted from each of the light-emitting elements is emitted from the semi-transmissive and semi-reflective electrode 402 side.

Further, strictly speaking, the total thickness from the reflective electrode 401 to the semi-transmissive and semi-reflective electrode 402 can be the total thickness from a reflection region in the reflective electrode 401 to a reflection region in the semi-transmissive and semi-reflective electrode 402. However, it is difficult to precisely determine the positions of the reflection regions in the reflective electrode 401 and the semi-transmissive and semi-reflective electrode 402; therefore, it is presumed that the above effect can be sufficiently obtained wherever the reflection regions may be set in the reflective electrode 401 and the semi-transmissive and semi-reflective electrode 402.

Next, in the first light-emitting element (R) 410R, the optical path length from the reflective electrode 401 to the third light-emitting layer (R) 404R is adjusted to a desired thickness ((2 m′+1)λ_(R)/4, where m′ is a natural number); thus, light emitted from the third light-emitting layer (R) 404R can be amplified. Light (first reflected light) that is reflected by the reflective electrode 401 of the light emitted from the third light-emitting layer (R) 404R interferes with light (first incident light) that directly enters the semi-transmissive and semi-reflective electrode 402 from the third light-emitting layer (R) 404R. Therefore, by adjusting the optical path length from the reflective electrode 401 to the third light-emitting layer (R) 404R to the desired value ((2 m′+1)λ_(R)/4, where m′ is a natural number), the phases of the first reflected light and the first incident light can be aligned with each other and the light emitted from the third light-emitting layer (R) 404R can be amplified.

Note that strictly speaking, the optical path length from the reflective electrode 401 to the third light-emitting layer (R) 404R can be the optical path length from a reflection region in the reflective electrode 401 to a light-emitting region in the third light-emitting layer (R) 404R. However, it is difficult to precisely determine the positions of the reflection region in the reflective electrode 401 and the light-emitting region in the third light-emitting layer (R) 404R; therefore, it is presumed that the above effect can be sufficiently obtained wherever the reflection region and the light-emitting region may be set in the reflective electrode 401 and the third light-emitting layer (R) 404R, respectively.

Next, in the second light-emitting element (G) 410G, the optical path length from the reflective electrode 401 to the second light-emitting layer (G) 404G is adjusted to a desired thickness ((2 m″+1)λ_(G)/4, where m″ is a natural number); thus, light emitted from the second light-emitting layer (G) 404G can be amplified. Light (second reflected light) that is reflected by the reflective electrode 401 of the light emitted from the second light-emitting layer (G) 404G interferes with light (second incident light) that directly enters the semi-transmissive and semi-reflective electrode 402 from the second light-emitting layer (G) 404G. Therefore, by adjusting the optical path length from the reflective electrode 401 to the second light-emitting layer (G) 404G to the desired value ((2 m″+1)λ_(G)/4, where m″ is a natural number), the phases of the second reflected light and the second incident light can be aligned with each other and the light emitted from the second light-emitting layer (G) 404G can be amplified.

Note that strictly speaking, the optical path length from the reflective electrode 401 to the second light-emitting layer (G) 404G can be the optical path length from a reflection region in the reflective electrode 401 to a light-emitting region in the second light-emitting layer (G) 404G. However, it is difficult to precisely determine the positions of the reflection region in the reflective electrode 401 and the light-emitting region in the second light-emitting layer (G) 404G; therefore, it is presumed that the above effect can be sufficiently obtained wherever the reflection region and the light-emitting region may be set in the reflective electrode 401 and the second light-emitting layer (G) 404G, respectively.

Next, in the third light-emitting element (B) 410B, the optical path length from the reflective electrode 401 to the first light-emitting layer (B) 404B is adjusted to a desired thickness ((2 m′″+1)λ_(B)/4, where m′″ is a natural number); thus, light emitted from the first light-emitting layer (B) 404B can be amplified. Light (third reflected light) that is reflected by the reflective electrode 401 of the light emitted from the first light-emitting layer (B) 404B interferes with light (third incident light) that directly enters the semi-transmissive and semi-reflective electrode 402 from the first light-emitting layer (B) 404B. Therefore, by adjusting the optical path length from the reflective electrode 401 to the first light-emitting layer (B) 404B to the desired value ((2 m′″+1)λ_(B)/4, where m′″ is a natural number), the phases of the third reflected light and the third incident light can be aligned with each other and the light emitted from the first light-emitting layer (B) 404B can be amplified.

Note that strictly speaking, the optical path length from the reflective electrode 401 to the first light-emitting layer (B) 404B in the third light-emitting element can be the optical path length from a reflection region in the reflective electrode 401 to a light-emitting region in the first light-emitting layer (B) 404B. However, it is difficult to precisely determine the positions of the reflection region in the reflective electrode 401 and the light-emitting region in the first light-emitting layer (B) 404B; therefore, it is presumed that the above effect can be sufficiently obtained wherever the reflection region and the light-emitting region may be set in the reflective electrode 401 and the first light-emitting layer (B) 404B, respectively.

Note that although each of the light-emitting elements in the above-described structure includes a plurality of light-emitting layers in the EL layer, the present invention is not limited thereto; for example, the structure of the tandem light-emitting element which is described in Embodiment 3 can be combined, in which case a plurality of EL layers and a charge generation layer interposed therebetween are provided in one light-emitting element and one or more light-emitting layers are formed in each of the EL layers.

The light-emitting device described in this embodiment has a microcavity structure, in which light with wavelengths which differ depending on the light-emitting elements can be extracted even when they include the same EL layer, so that it is not needed to form light-emitting elements for the colors of R, G, and B. Therefore, the above structure is advantageous for full color display owing to easiness in achieving higher resolution display or the like. Note that a combination with coloring layers (color filters) is also possible. In addition, emission intensity with a predetermined wavelength in the front direction can be increased, whereby power consumption can be reduced. The above structure is particularly useful in the case of being applied to a color display (image display device) including pixels of three or more colors but may also be applied to lighting or the like.

(Embodiment 5)

In this embodiment, a light-emitting device including a light-emitting element which is one embodiment of the present invention will be described.

The light-emitting device can be either a passive matrix light-emitting device or an active matrix light-emitting device. Note that any of the light-emitting elements described in the other embodiments can be applied to the light-emitting device described in this embodiment.

In this embodiment, an active matrix light-emitting device is described with reference to FIGS. 8A and 8B.

Note that FIG. 8A is a top view illustrating a light-emitting device and FIG. 8B is a cross-sectional view taken along the chain line A-A′ in FIG. 8A. The active matrix light-emitting device according to this embodiment includes a pixel portion 502 provided over an element substrate 501, a driver circuit portion (a source line driver circuit) 503, and driver circuit portions (gate line driver circuits) 504 (504 a and 504 b). The pixel portion 502, the driver circuit portion 503, and the driver circuit portions 504 are sealed between the element substrate 501 and a sealing substrate 506 with a sealant 505.

In addition, there is provided a lead wiring 507 over the element substrate 501. The lead wiring 507 is provided for connecting an external input terminal through which a signal (e.g., a video signal, a clock signal, a start signal, or a reset signal) or a potential from the outside is transmitted to the driver circuit portion 503 and the driver circuit portions 504. Here is shown an example in which a flexible printed circuit (FPC) 508 is provided as the external input terminal. Although only the FPC is illustrated, this FPC may be provided with a printed wiring board (PWB). The light-emitting device in this specification includes, in its category, not only the light-emitting device itself but also the light-emitting device provided with the FPC or the PWB.

Next, a cross-sectional structure is described with reference to FIG. 8B. The driver circuit portions and the pixel portion are formed over the element substrate 501; here are illustrated the driver circuit portion 503 which is the source line driver circuit and the pixel portion 502.

The driver circuit portion 503 is an example where a CMOS circuit is formed, which is a combination of an n-channel TFT 509 and a p-channel TFT 510. Note that a circuit included in the driver circuit portion may be formed using any of various circuits, such as a CMOS circuit, a PMOS circuit, or an NMOS circuit. Although a driver-integrated type in which a driver circuit is formed over the substrate is described in this embodiment, the present invention is not limited to this type, and the driver circuit can be formed outside the substrate.

The pixel portion 502 includes a plurality of pixels each of which includes a switching TFT 511, a current control TFT 512, and a first electrode (anode) 513 which is electrically connected to a wiring (a source electrode or a drain electrode) of the current control TFT 512. Note that an insulator 514 is formed to cover end portions of the first electrode (anode) 513. In this embodiment, the insulator 514 is formed using a positive photosensitive acrylic resin.

The insulator 514 preferably has a curved surface with curvature at an upper end portion or a lower end portion thereof in order to obtain favorable coverage by a film which is to be stacked over the insulator 514. For example, in the case of using a positive photosensitive acrylic resin as a material for the insulator 514, the insulator 514 preferably has a curved surface with a curvature radius (0.2 μm to 3 μm) at the upper end portion. The insulator 514 can be formed using either a negative photosensitive resin or a positive photosensitive resin. It is possible to use, without limitation to an organic compound, either an organic compound or an inorganic compound such as silicon oxide or silicon oxynitride.

An EL layer 515 and a second electrode (cathode) 516 are stacked over the first electrode (anode) 513. In the EL layer 515, at least a light-emitting layer is provided. The light-emitting layer has such a stacked-layer structure as described in Embodiment 1. Further, in the EL layer 515, a hole-injection layer, a hole-transport layer, an electron-transport layer, an electron-injection layer, a charge generation layer, and the like can be provided as appropriate in addition to the light-emitting layer.

The stacked-layer structure including the first electrode (anode) 513, the EL layer 515, and the second electrode (cathode) 516 forms a light-emitting element 517. For the first electrode (anode) 513, the EL layer 515, and the second electrode (cathode) 516, the materials described in Embodiment 2 can be used. Although not illustrated, the second electrode (cathode) 516 is electrically connected to the FPC 508 which is an external input terminal.

Although the cross-sectional view in FIG. 8B illustrates only one light-emitting element 517, a plurality of light-emitting elements is arranged in a matrix in the pixel portion 502. Light-emitting elements which provide three kinds of light emission (R, G, and B) are selectively formed in the pixel portion 502, whereby a light-emitting device capable of full color display can be fabricated. Alternatively, a light-emitting device capable of full color display may be fabricated by a combination with coloring layers (color filters).

Further, the sealing substrate 506 is attached to the element substrate 501 with the sealant 505, whereby the light-emitting element 517 is provided in a space 518 surrounded by the element substrate 501, the sealing substrate 506, and the sealant 505. The space 518 may be filled with an inert gas (such as nitrogen or argon) or the sealant 505.

An epoxy-based resin, low-melting-point glass, or the like is preferably used for the sealant 505. It is preferable that such a material do not transmit moisture or oxygen as much as possible. As the sealing substrate 506, a glass substrate, a quartz substrate, or a plastic substrate formed of fiberglass reinforced plastic (FRP), polyvinyl fluoride) (PVF), polyester, acrylic, or the like can be used.

As described above, an active matrix light-emitting device can be obtained.

Note that the structure described in this embodiment can be combined as appropriate with any of the structures described in the other embodiments.

(Embodiment 6)

In this embodiment, examples of a variety of electronic devices which are completed using a light-emitting device will be described with reference to FIGS. 9A to 9D and FIGS. 10A to 10C. The light-emitting device is fabricated using a light-emitting element which is one embodiment of the present invention.

Examples of electronic devices to which the light-emitting device is applied are television devices (also referred to as TV or television receivers), monitors for computers and the like, cameras such as digital cameras and digital video cameras, digital photo frames, cellular phones (also referred to as portable telephone devices), portable game machines, portable information terminals, audio playback devices, large game machines such as pin-ball machines, and the like. Specific examples of these electronic devices are illustrated in FIGS. 9A to 9D.

FIG. 9A illustrates an example of a television device. In a television device 7100, a display portion 7103 is incorporated in a housing 7101. The display portion 7103 is capable of displaying images, and a light-emitting device can be used for the display portion 7103. In addition, here, the housing 7101 is supported by a stand 7105.

The television device 7100 can be operated with an operation switch provided in the housing 7101 or a separate remote controller 7110. With operation keys 7109 of the remote controller 7110, channels and volume can be controlled and images displayed on the display portion 7103 can be controlled. Furthermore, the remote controller 7110 may be provided with a display portion 7107 for displaying data output from the remote controller 7110.

Note that the television device 7100 is provided with a receiver, a modem, and the like. With the receiver, general television broadcasting can be received. Furthermore, when the television device 7100 is connected to a communication network by wired or wireless connection via the modem, one-way (from a transmitter to a receiver) or two-way (between a transmitter and a receiver, between receivers, or the like) data communication can be performed.

FIG. 9B illustrates a computer, which includes a main body 7201, a housing 7202, a display portion 7203, a keyboard 7204, an external connection port 7205, a pointing device 7206, and the like. Note that this computer is manufactured by using a light-emitting device for the display portion 7203.

FIG. 9C illustrates a portable game machine, which includes two housings, i.e., a housing 7301 and a housing 7302, connected to each other via a joint portion 7303 so that the portable game machine can be opened or closed. A display portion 7304 is incorporated in the housing 7301 and a display portion 7305 is incorporated in the housing 7302. In addition, the portable game machine illustrated in FIG. 9C includes a speaker portion 7306, a recording medium insertion portion 7307, an LED lamp 7308, input means (an operation key 7309, a connection terminal 7310, a sensor 7311 (a sensor having a function of measuring force, displacement, position, speed, acceleration, angular velocity, rotational frequency, distance, light, liquid, magnetism, temperature, chemical substance, sound, time, hardness, electric field, electric current, voltage, electric power, radiation, flow rate, humidity, gradient, oscillation, odor, or infrared rays), and a microphone 7312), and the like. It is needless to say that the structure of the portable game machine is not limited to the above structure as long as a light-emitting device is used for at least either the display portion 7304 or the display portion 7305, or both, and may include other accessories as appropriate. The portable game machine illustrated in FIG. 9C has a function of reading out a program or data stored in a storage medium to display it on the display portion, and a function of sharing information with another portable game machine by wireless communication. Note that the portable game machine illustrated in FIG. 9C can have a variety of functions without limitation to those above.

FIG. 9D illustrates an example of a cellular phone. A cellular phone 7400 is provided with a display portion 7402 incorporated in a housing 7401, operation buttons 7403, an external connection port 7404, a speaker 7405, a microphone 7406, and the like. Note that the cellular phone 7400 is manufactured using a light-emitting device for the display portion 7402.

When the display portion 7402 of the cellular phone 7400 illustrated in FIG. 9D is touched with a finger or the like, data can be input to the cellular phone 7400. Further, operations such as making a call and creating e-mail can be performed by touch on the display portion 7402 with a finger or the like.

There are mainly three screen modes for the display portion 7402. The first mode is a display mode mainly for displaying an image. The second mode is an input mode mainly for inputting information such as characters. The third mode is a display-and-input mode in which two modes of the display mode and the input mode are mixed.

For example, in the case of making a call or creating e-mail, a character input mode mainly for inputting characters is selected for the display portion 7402 so that characters displayed on the screen can be input. In this case, it is preferable to display a keyboard or number buttons on almost the entire screen of the display portion 7402.

When a detection device including a sensor for detecting inclination, such as a gyroscope or an acceleration sensor, is provided inside the cellular phone 7400, display on the screen of the display portion 7402 can be automatically changed by determining the orientation of the cellular phone 7400 (whether the cellular phone is placed horizontally or vertically for a landscape mode or a portrait mode).

The screen modes are changed by touch on the display portion 7402 or operation with the operation buttons 7403 of the housing 7401. Alternatively, the screen modes can be changed depending on the kind of image displayed on the display portion 7402. For example, when a signal for an image to be displayed on the display portion is data of moving images, the screen mode is changed to the display mode. When the signal is text data, the screen mode is changed to the input mode.

Moreover, in the input mode, if a signal detected by an optical sensor in the display portion 7402 is detected and the input by touch on the display portion 7402 is not performed for a certain period, the screen mode may be controlled so as to be changed from the input mode to the display mode.

The display portion 7402 may function as an image sensor. For example, an image of a palm print, a fingerprint, or the like is taken by touch on the display portion 7402 with the palm or the finger, whereby personal identification can be performed. Furthermore, when a backlight or a sensing light source which emits near-infrared light is provided for the display portion, an image of a finger vein, a palm vein, or the like can also be taken.

FIGS. 10A and 10B illustrate a foldable tablet terminal. The tablet terminal is opened in FIG. 10A. The tablet terminal includes a housing 9630, a display portion 9631 a, a display portion 9631 b, a display mode switch 9034, a power switch 9035, a power saver switch 9036, a clasp 9033, and an operation switch 9038. The tablet terminal is manufactured using the light-emitting device for either the display portion 9631 a or the display portion 9631 b or both.

Part of the display portion 9631 a can be a touch panel region 9632 a and data can be input when a displayed operation key 9637 is touched. Although a structure in which a half region in the display portion 9631 a has only a display function and the other half region also has a touch panel function is shown as an example, the display portion 9631 a is not limited to the structure. The whole region in the display portion 9631 a may have a touch panel function. For example, the display portion 9631 a can display keyboard buttons in the whole region to be a touch panel, and the display portion 9631 b can be used as a display screen.

As in the display portion 9631 a, part of the display portion 9631 b can be a touch panel region 9632 b. When a keyboard display switching button 9639 displayed on the touch panel is touched with a finger, a stylus, or the like, a keyboard can be displayed on the display portion 9631 b.

Touch input can be performed in the touch panel region 9632 a and the touch panel region 9632 b at the same time.

The display mode switch 9034 can switch the display between portrait mode, landscape mode, and the like, and between monochrome display and color display, for example. The power saver switch 9036 can control display luminance in accordance with the amount of external light in use of the tablet terminal detected by an optical sensor incorporated in the tablet terminal In addition to the optical sensor, another detection device including a sensor for detecting inclination, such as a gyroscope or an acceleration sensor, may be incorporated in the tablet terminal.

FIG. 10A shows an example in which the display portion 9631 a and the display portion 9631 b have the same display area; however, without limitation thereon, one of the display portions may be different from the other display portion in size and display quality. For example, one display panel may be capable of higher-definition display than the other display panel.

The tablet terminal is closed in FIG. 10B. The tablet terminal includes the housing 9630, a solar cell 9633, a charge and discharge control circuit 9634, a battery 9635, and a DCDC converter 9636. In FIG. 10B, a structure including the battery 9635 and the DCDC converter 9636 is illustrated as an example of the charge and discharge control circuit 9634.

Since the tablet terminal is foldable, the housing 9630 can be closed when the tablet terminal is not used. As a result, the display portion 9631 a and the display portion 9631 b can be protected; thus, a tablet terminal which has excellent durability and excellent reliability in terms of long-term use can be provided.

In addition, the tablet terminal illustrated in FIGS. 10A and 10B can have a function of displaying a variety of kinds of data (e.g., a still image, a moving image, and a text image), a function of displaying a calendar, a date, the time, or the like on the display portion, a touch-input function of operating or editing the data displayed on the display portion by touch input, a function of controlling processing by a variety of kinds of software (programs), and the like.

The solar cell 9633 provided on a surface of the tablet terminal can supply power to the touch panel, the display portion, a video signal processing portion, or the like. Note that the solar cell 9633 can be provided on one or both surfaces of the housing 9630 and the battery 9635 can be charged efficiently. The use of a lithium ion battery as the battery 9635 is advantageous in downsizing or the like.

The structure and the operation of the charge and discharge control circuit 9634 illustrated in FIG. 10B will be described with reference to a block diagram in FIG. 10C. The solar cell 9633, the battery 9635, the DCDC converter 9636, a converter 9638, switches SW1 to SW3, and a display portion 9631 are illustrated in FIG. 10C, and the battery 9635, the DCDC converter 9636, the converter 9638, and the switches SW1 to SW3 correspond to the charge and discharge control circuit 9634 illustrated in FIG. 10B.

First, an example of the operation in the case where power is generated by the solar cell 9633 using external light is described. The voltage of power generated by the solar cell 9633 is stepped up or down by the DCDC converter 9636 so that the power has a voltage for charging the battery 9635. Then, when the power from the solar cell 9633 is used for the operation of the display portion 9631, the switch SW1 is turned on and the voltage of the power is stepped up or down by the converter 9638 so as to be a voltage needed for the display portion 9631. In addition, when display on the display portion 9631 is not performed, the switch SW1 is turned off and the switch SW2 is turned on so that the battery 9635 may be charged.

Note that the solar cell 9633 is described as an example of a power generation means; however, without limitation thereon, the battery 9635 may be charged using another power generation means such as a piezoelectric element or a thermoelectric conversion element (Peltier element). For example, a non-contact electric power transmission module which transmits and receives power wirelessly (without contact) to charge the battery 9635, or a combination of the solar cell 9633 and another means for charge may be used.

It is needless to say that an embodiment of the present invention is not limited to the electronic device illustrated in FIGS. 10A to 10C as long as the display portion described in the above embodiment is included.

As described above, the electronic devices can be obtained by the use of the light-emitting device which is one embodiment of the present invention. The light-emitting device has a remarkably wide application range, and can be applied to electronic devices in a variety of fields.

Note that the structure described in this embodiment can be combined as appropriate with any of the structures described in the other embodiments.

(Embodiment 7)

In this embodiment, examples of lighting devices will be described with reference to FIG. 11. A light-emitting device including a light-emitting element which is one embodiment of the present invention is applied to the lighting devices.

FIG. 11 illustrates an example in which a light-emitting device is used for an interior lighting device 8001. Since the light-emitting device can have a larger area, a lighting device having a large area can also be formed. In addition, a lighting device 8002 in which a light-emitting region has a curved surface can also be formed with the use of a housing with a curved surface. A light-emitting element included in the light-emitting device described in this embodiment is in the form of a thin film, which allows the housing to be designed more freely. Therefore, the lighting device can be elaborately designed in a variety of ways. Further, a wall of the room may be provided with a large-sized lighting device 8003.

Moreover, when the light-emitting device is used for a table by being used as a surface of a table, a lighting device 8004 which has a function as a table can be obtained. When the light-emitting device is used as part of other furniture, a lighting device which has a function as the furniture can be obtained.

In this manner, a variety of lighting devices to which the light-emitting device is applied can be obtained. Note that such lighting devices are also embodiments of the present invention.

The structure described in this embodiment can be combined as appropriate with any of the structures described in the other embodiments.

EXAMPLE 1

In this example, a light-emitting element 1 which is one embodiment of the present invention is described with reference to FIG. 12. Chemical formulae of materials used in this example are shown below.

<<Fabrication of Light-Emitting Element 1>>

First, a film of indium tin oxide containing silicon oxide (ITSO) was formed over a glass substrate 1100 by a sputtering method, so that a first electrode 1101 functioning as an anode was formed. Note that the thickness was set to 110 nm and the electrode area was set to 2 mm×2 mm.

Next, as pretreatment for forming the light-emitting element 1 over the substrate 1100, UV ozone treatment was performed for 370 seconds after washing of a surface of the substrate with water and baking that was performed at 200° C. for one hour.

After that, the substrate was transferred into a vacuum evaporation apparatus where the pressure had been reduced to approximately 10⁻⁴ Pa, and was subjected to vacuum baking at 170° C. for 30 minutes in a heating chamber of the vacuum evaporation apparatus, and then the substrate 1100 was cooled down for about 30 minutes.

Next, the substrate 1100 was fixed to a holder in the vacuum evaporation apparatus so that a surface on which the first electrode 1101 was provided faced downward. In this example, a case is described in which a hole-injection layer 1111, a hole-transport layer 1112, a light-emitting layer 1113, an electron-transport layer 1114, and an electron-injection layer 1115 which are included in an EL layer 1102 are sequentially formed by a vacuum evaporation method.

The pressure in the vacuum evaporation apparatus was reduced to about 10⁻⁴ Pa. Then, 1,3,5-tri(dibenzothiophen-4-yl)benzene (abbreviation: DBT3P-II) and molybdenum(VI) oxide were co-evaporated with a mass ratio of DBT3P-II (abbreviation) to molybdenum oxide being 4:2, whereby the hole-injection layer 1111 was formed over the first electrode 1101. The thickness of the hole-injection layer 1111 was set to 20 nm. Note that co-evaporation is an evaporation method by which a plurality of different substances is concurrently vaporized from respective different evaporation sources.

Next, the hole-transport layer 1112 was formed by evaporation of 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine (abbreviation: BPAFLP) to a thickness of 20 nm.

Next, the light-emitting layer 1113 was formed over the hole-transport layer 1112. The light-emitting layer 1113 having a stacked-layer structure was formed by forming a first light-emitting layer 1113 a with a thickness of 20 nm by co-evaporation of 4,6-bis[3-(9H-carbazol-9-yl)phenyl]pyrimidine (abbreviation: 4,6mCzP2 Pm), N-(1,1′-biphenyl-4-yl)-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-9,9-dimethyl-9H-fluor en-2-amine (abbreviation: PCBBiF), and tris(2-phenylpyridinato)iridium(III) (abbreviation: [Ir(ppy)₃]) with a mass ratio of 4,6mCzP2 Pm (abbreviation) to PCBBiF (abbreviation) and [Ir(ppy)₃] (abbreviation) being 0.7:0.3:0.05, and then forming a second light-emitting layer 1113 b with a thickness of 20 nm by co-evaporation of 2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[O]quinoxaline (abbreviation: 2mDBTBPDBq-II), PCBBiF (abbreviation), and bis{4,6-dimethyl-2-[5-(2,6-dimethylphenyl)-3-(3,5-dimethylphenyl)-2-pyrazinyl-κN]phenyl-κC}(2,8-dimethyl-4,6-nonanedionato-κ²O,O′)iridium(III) (abbreviation: [Ir(dmdppr-dmp)₂(divm)]) with a mass ratio of 2mDBTBPDBq-II (abbreviation) to PCBBiF (abbreviation) and [Ir(dmdppr-dmp)₂(divm)] (abbreviation) being 0.9:0.1:0.05.

Next, over the light-emitting layer 1113, the electron-transport layer 1114 was formed in such a manner that a film of 2mDBTBPDBq-II (abbreviation) was formed by evaporation to a thickness of 15 nm and then a film of bathophenanthroline (abbreviation: Bphen) was formed by evaporation to a thickness of 10 nm. Further, over the electron-transport layer 1114, a film of lithium fluoride was formed by evaporation to a thickness of 1 nm to form the electron-injection layer 1115.

Lastly, over the electron-injection layer 1115, an aluminum film was formed by evaporation to a thickness of 200 nm as a second electrode 1103 functioning as a cathode. Thus, the light-emitting element 1 was fabricated. Note that in all the above evaporation steps, evaporation was performed by a resistance-heating method.

Table 1 shows an element structure of the light-emitting element 1 obtained as described above.

TABLE 1 Hole- Hole- Light- Electron- First injection transport emitting Electron-transport injection Second Electrode Layer Layer Layer Layer Layer Electrode Light- ITSO DBT3P-II: BPAFLP * ** 2mDBTBPDBq- Bphen LiF Al emitting (110 nm) MoO_(x) (20 nm) II (10 nm) (1 nm) (200 nm) Element 1 (4:2 20 nm) (15 nm) * 4,6mCzP2Pm:PCBBiF:[Ir(ppy)₃] (0.7:0.3:0.05 20 nm) ** 2mDBTBPDBq-II:PCBBiF:[Ir(dmdppr—dmp)₂(divm)] (0.9:0.1:0.05 20 nm)

The fabricated light-emitting element 1 was sealed in a glove box containing a nitrogen atmosphere so as not to be exposed to air (specifically, a sealant was applied to an outer edge of the element and heat treatment was performed at 80° C. for 1 hour at the time of sealing).

<<Operation Characteristics of Light-emitting Element 1>>

Operation characteristics of the fabricated light-emitting element 1 were measured. Note that the measurements were carried out at room temperature (in an atmosphere kept at 25° C.).

First, FIG. 13 shows current density-luminance characteristics of the light-emitting element 1. In FIG. 13, the vertical axis represents luminance (cd/m²), and the horizontal axis represents current density (mA/cm²). FIG. 14 shows voltage-luminance characteristics of the light-emitting element 1. In FIG. 14, the vertical axis represents luminance (cd/m²), and the horizontal axis represents voltage (V). FIG. 15 shows luminance-current efficiency characteristics of the light-emitting element 1. In FIG. 15, the vertical axis represents current efficiency (cd/A), and the horizontal axis represents luminance (cd/m²). FIG. 16 shows voltage-current characteristics of the light-emitting element 1. In FIG. 16, the vertical axis represents current (mA), and the horizontal axis represents voltage (V).

FIG. 15 reveals high efficiency of the light-emitting element 1 that is one embodiment of the present invention. Table 2 below shows initial values of main characteristics of the light-emitting element 1 at a luminance of about 1000 cd/m².

TABLE 2 External Current Current Power Quantum Voltage Current Density Chromaticity Luminance Efficiency Efficiency Efficiency (V) (mA) (mA/cm²) (x, y) (cd/m²) (cd/A) (lm/W) (%) Light- 3.2 0.056 1.4 (0.48, 0.50) 1000 72 70 27 emitting Element 1

The above results show that the light-emitting element 1 fabricated in this example has high external quantum efficiency, which means its high emission efficiency.

FIG. 17 shows an emission spectrum when a current at a current density of 25 mA/cm² was supplied to the light-emitting element 1. As shown in FIG. 17, the emission spectrum of the light-emitting element 1 has two peaks at around 519 nm and 615 nm, which suggests that the two peaks are respectively derived from emission from the phosphorescent organometallic iridium complexes [Ir(ppy)₃] and [Ir(dmdppr-dmp)₂(divm)].

Note that by cyclic voltammetry, the LUMO level of 4,6mCzP2 Pm, which is the first organic compound with an electron-transport property, was estimated to be −2.88 eV, and the LUMO level of 2mDBTBPDBq-II, which is the third organic compound with an electron-transport property, was estimated to be −2.94 eV. Thus, the first organic compound has a higher LUMO level than the third organic compound.

Here, FIG. 18 shows an emission spectrum of a thin film of 4,6mCzP2 Pm, which is the first organic compound with an electron-transport property, an emission spectrum of a thin film of PCBBiF, which is the second organic compound with a hole-transport property, and an emission spectrum of a mixed film (co-evaporation film) of 4,6mCzP2 Pm and PCBBiF. Further, FIG. 19 shows an emission spectrum of a thin film of 2mDBTBPDBq-II, which is the third organic compound with an electron-transport property, an emission spectrum of a thin film of PCBBiF, which is the second organic compound with a hole-transport property, and an emission spectrum of a mixed film (co-evaporation film) of 2mDBTBPDBq-II and PCBBiF.

In each of FIG. 18 and FIG. 19, the emission wavelength of the mixed film is located on a longer wavelength side with respect to the emission wavelengths of the single films of the materials, which indicates that the first organic compound (4,6mCzP2 Pm) and the second organic compound (PCBBiF) formed an exciplex and the third organic compound (2mDBTBPDBq-II) and the second organic compound (PCBBiF) formed an exciplex.

This application is based on Japanese Patent Application serial no. 2012-091615 filed with Japan Patent Office on Apr. 13, 2012 and Japanese Patent Application serial no. 2013-048051 filed with Japan Patent Office on Mar. 11, 2013, the entire contents of which are hereby incorporated by reference. 

What is claimed is:
 1. A light-emitting element comprising: a light-emitting layer between an anode and a cathode, wherein the light-emitting layer is a stack of a first light-emitting layer and a second light-emitting layer, wherein the first light-emitting layer is located at the anode side, wherein the first light-emitting layer comprises a first light-emitting substance converting triplet excitation energy into light emission, a first organic compound having an electron-transport property, and a second organic compound having a hole-transport property, wherein the second light-emitting layer comprises a second light-emitting substance converting triplet excitation energy into light emission, the second organic compound, and a third organic compound having an electron-transport property, wherein the first organic compound has a higher lowest unoccupied molecular orbital level than the third organic compound, wherein the first light-emitting substance emits light with a wavelength shorter than a wavelength of light emitted from the second light-emitting substance, wherein the first organic compound and the second organic compound form a first exciplex, wherein the second organic compound and the third organic compound form a second exciplex, wherein an emission spectrum of the first exciplex is overlapped with an absorption spectrum of the first light-emitting substance, and wherein light emission from the first exciplex is substantially not observed when the first light-emitting layer emits a first light.
 2. The light-emitting element according to claim 1, wherein the first exciplex has higher excitation energy than the second exciplex.
 3. The light-emitting element according to claim 1, wherein the first exciplex is formed from an anion of the first organic compound and a cation of the second organic compound, and wherein the second exciplex is formed from an anion of the third organic compound and a cation of the second organic compound.
 4. The light-emitting element according to claim 1, wherein the first light-emitting substance is a phosphorescent compound or a thermally activated delayed fluorescence material, wherein the second light-emitting substance is a phosphorescent compound or a thermally activated delayed fluorescence material, wherein the first organic compound is a π-electron deficient heteroaromatic compound, wherein the second organic compound is a π-electron rich heteroaromatic compound or an aromatic amine compound, and wherein the third organic compound is a π-electron deficient heteroaromatic compound.
 5. A light-emitting device comprising the light-emitting element according to claim
 1. 6. An electronic device comprising the light-emitting device according to claim
 5. 7. A lighting device comprising the light-emitting device according to claim
 5. 8. A light-emitting element comprising: a light-emitting layer between an anode and a cathode; a hole-transport layer between the anode and the light-emitting layer; and an electron-transport layer between the cathode and the light-emitting layer, wherein the light-emitting layer is a stack of a first light-emitting layer and a second light-emitting layer, wherein the first light-emitting layer is in contact with the hole-transport layer, wherein the first light-emitting layer comprises a first light-emitting substance converting triplet excitation energy into light emission, a first organic compound having an electron-transport property, and a second organic compound having a hole-transport property, wherein the second light-emitting layer is in contact with the electron-transport layer, wherein the second light-emitting layer comprises a second light-emitting substance converting triplet excitation energy into light emission, the second organic compound, and a third organic compound having an electron-transport property, wherein the first organic compound has a higher lowest unoccupied molecular orbital level than the third organic compound, wherein the first light-emitting substance emits light with a wavelength shorter than a wavelength of light emitted from the second light-emitting substance, wherein the first organic compound and the second organic compound form a first exciplex, wherein the second organic compound and the third organic compound form a second exciplex, wherein an emission spectrum of the first exciplex is overlapped with an absorption spectrum of the first light-emitting substance, and wherein light emission from the first exciplex is substantially not observed when the first light-emitting layer emits a first light.
 9. The light-emitting element according to claim 8, wherein the first exciplex has higher excitation energy than the second exciplex.
 10. The light-emitting element according to claim 8, wherein the first exciplex is formed from an anion of the first organic compound and a cation of the second organic compound, and wherein the second exciplex is formed from an anion of the third organic compound and a cation of the second organic compound.
 11. The light-emitting element according to claim 8, wherein the first light-emitting substance is a phosphorescent compound or a thermally activated delayed fluorescence material, wherein the second light-emitting substance is a phosphorescent compound or a thermally activated delayed fluorescence material, wherein the first organic compound is a π-electron deficient heteroaromatic compound, wherein the second organic compound is a π-electron rich heteroaromatic compound or an aromatic amine compound, and wherein the third organic compound is a π-electron deficient heteroaromatic compound.
 12. A light-emitting device comprising the light-emitting element according to claim
 8. 13. An electronic device comprising the light-emitting device according to claim
 12. 14. A lighting device comprising the light-emitting device according to claim
 12. 15. A light-emitting element comprising: an anode: a first light-emitting layer over the anode; a second light-emitting layer over the first light-emitting layer; and a cathode over the second light-emitting layer, wherein the first light-emitting layer comprises 4,6-bis[3-(9H-carbazol-9-yl)phenyl]pyrimidine, N-(1,1′-biphenyl-4-yl)-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-9,9-dimethyl-9H-fluoren-2-amine, and a first light-emitting substance converting triplet excitation energy into light emission, wherein the second light-emitting layer comprises 2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline, N-(1,1′-biphenyl-4-yl)-N-[4-(9-phenyl-9H -carbazol-3-yl)phenyl]-9,9-dimethyl-9H-fluoren-2-amine, and a second light-emitting substance converting triplet excitation energy into light emission, wherein the 4,6-bis[3-(9H-carbazol-9-yl)phenyl]pyrimidine and the N-(1,1′-biphenyl-4-yl)-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-9,9-dimethyl-9H-fluoren-2-amine form a first exciplex, wherein the 2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline and the N-(1,1′-biphenyl-4-yl)-N[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-9,9-dimethyl-9H -fluoren-2-amine form a second exciplex, wherein an emission spectrum of the first exciplex is overlapped with an absorption spectrum of the first light-emitting substance, and wherein light emission from the first exciplex is substantially not observed when the first light-emitting layer emits a first light.
 16. The light-emitting element according to claim 15, wherein the first light-emitting substance is tris(2-phenylpyridinato)iridium(III).
 17. The light-emitting element according to claim 15, wherein the second light-emitting substance is bis{4,6-dimethyl-2-[5-(2,6-dimethylphenyl)-3-(3,5-dimethylphenyl)-2-pyrazinyl-κN]phenyl-κC}(2,8-dimethyl-4,6-nonanedionato -κ²O,O′)iridium(III).
 18. A light-emitting device comprising the light-emitting element according to claim
 15. 19. An electronic device comprising the light-emitting device according to claim
 18. 20. A lighting device comprising the light-emitting device according to claim
 18. 